My computer is accumulating a lot of dust though I use a very good casing .I feel it has slowed down a lot .  I am thinking of getting air filters after doing online research but I think since air filters and room air purifiers are the same maybe I can use my room air purifier (http://go-clair.com/)  and place it next to the casing .

 Do you think it is a good idea ? I don't know how to clean using compressed air etc . I use a hair dryer ,vacuum cleaner to blow out the dust and give it to the computer cleaning technician once a year . I've never used a room air purifier so I can't comment whether that will help with dust in general.
Hair dryers and vacuum cleaners shouldn't be used to clean computers due to the potential for causing problems.  To clean with compressed air, turn the machine off, remove the side of the case, then give it a good spray inside to blow the dust out.  Main areas to pay attention to are any fans (which can also be gently wiped with e.g. kitchen towel), the CPU heatsink, and any fan filters or intakes which are generally at the front of the case.  I live in a very dusty flat as it has really high ceilings, I tend to wipe down the front of my PC every few days to remove dust building up there, remove the fan filters for cleaning every few weeks, and give the inside a good clean out with compressed air and kitchen towel every month or two.I have installed computers in clean air cabinets before, but its kind of overkill for a home computer. Situations where clean air cabinets were used were in industrial and automotive garages where the main risk is OIL particles and metal dust that can cause havoc. The oil collects on the blades of fans and makes a STICKY adhesive to gather dust particles that travel into the path of the fan blades. The problems with metal particulates is pretty obvious, a build up of metal dust can cause short circuits between component legs.

A air purification cabinet generally has an air tight seal so that the only air passing through it pass through the filter at one side of the cabinet and then the warmer exhausted air passes out the other side. Depending on how many microns the filter catches, the hardware inside the cabinet stays very clean for a long time. You then end up having to clean the filter for the cabinet on an interval basis to make sure that it doesnt get plugged up and stop all airflow causing risk of overheat to hardware in cabinet.

One of my friends once built this system that had 7 fans in it and LED glow bling. 2 at the front panel, 2 at the back panel 2 on top, and 1 large fan on the removable side panel. It was one of those ultra gamer PC cases for use with hardware that pumps out lots of heat and requires lots of airflow. His hardware in this case was a simple common Pentium 4 2.8Ghz HT build, not overclocked and a single GeForce 7600GT video card. he came to me complaining the one DAY that his system keeps filling up with dust inside and he has never had a system get so dirty so quickly. I took a look at what he had built and pretty much told him that your system is acting like a large vacuum cleaner and the inside of the case is acting like the bag. I suggested to him to disable some of the overkill of fans and it might not get filthy as quickly, so he went with my suggestion of only having one of the front panel fans on and 1 of the back panel fans on to make some airflow in and out but not turn the inside of the case into a violent vortex of air. Sure enough his system stayed plenty cool and instead of getting dirty within 30 days it now no longer had to be cleaned until every 6 months. So if you have overkill for computer case cooling fans you could disable some of them as long as the computer is able to stay cool with them disabled and not have it get dirty as quickly.

As far as using air filtration systems to get rid of dust in the air, I havent tried them to comment whether they work well or are just a waste of electricity.

Also location placement of the computer can affect how much dust it collects such as if it was moved on top of a desk vs resting on the floor where there is less dust to suck up on the desktop.

Lastly, I have been servicing other peoples computers for about 25 years and causes for lots of dust and computer failures have been caused by:

Pets that lay on or near the computer... Dogs that sleep on the floor near the computer that is also on the floor as well as cats that love to sleep on top of the computer tower where the warmth of the power supply warms the case and they love the warm surface to rest on. The hair and dander from the animals gets sucked into the systems. One persons system had a golden BEARD inside the front panel from shedded golden lab hair.

Smokers ... who smoke around their computers. The tar in the air coats the fan blades making them sticky and more prone to collecting dust and hair etc. As well as the cigarette ash gets sucked in as well. One customer had a computer that was only 9 months old that was literally an ash tray inside of it. It also didnt help that they rested their ash trays on top of the computer case and so it didnt have far for the ash to go to get sucked inside of the case which lead to an ash blanket of dust inside and an overheat of the chipset killing the motherboard.Yes, air filters help. An help reduce overall maintenance of any indoor area. The principal benefit is for people who are sensitive to dust mites and pollen. But as noted, such filters also remove smoke and dust particles and help keep things cleaner. As mentioned, tobacco smoke is is bad for electronic devices.
I have a redhead Maid... Quote

I have a redhead Maid...