Recently I just paid a shop to fit my new motherboard, CPU and graphics card. They SAID everything would be fine and that my motherboard setting would be transfered to my hard drive. But when I set everything up and turned it on, the computer started to beep and an error message saying "CPU fan fail or speed" (or something very similar. Then I reset my computer, it took me to a screen saying start windows in safe mode etc. Everytime I chose an option my computer would just reset and take me to the same screen again. I would contact the shop but at the MOMENT they are closed and i really want to try my comouter out.

My specs are:
Motherboard: Asus A8N-SLI SE
CPU: AMD Athlon 3700+
1.7gb ram
80gb hardrive
GPU: Nvidia geforce 7900 gt KO

Im not sure if it is something to do with the RAM or heating, when I felt inside the case everything was very hot. When i took my computer into the shops I asked for a new PSU becuase mine was only a 20 pins and the new motherboard was a 24 pin, but they said it would be ok with 20.I suspect just from the info you have provided they have not installed the proper PSU...trying to get it to run and or boot properly risks damaging that MBoard and CPU.
Take it back immediately and have them do it properly.

A 24 pin MBoard should have a 24 pin PSU.I know you're anxious to try it out but you'd better just wait until the opens and take it back to them and tell them it does not work. "everything was very hot" after just having the power on for a short time,NOT GOOD. And this matter of a 20 pin PSU with a motherboard with a 24-pin power connector does not sound right. I'm not sure on this point, so maybe someone else will comment. What is that shop telling you, that 4 pins on the motherboard connector can simply be left open? I don't think so.

Edit: Ok, I see Patio posted and commented on the pins issue.The guys at the shop definetly said that even that 20 pins would be fine even THOUGH it was 24. They say that 20 pins would be ok as long as I dont want anything more. I tried to boot it but everytime i tried it just resets at usua, do I just put my windows disc in my CD drive and press F8 and go to the boot menu? I guess I'll just have to wait for the shop to open and ring them Better educate yourself some more on the # of pins issue; here's some info: Do 20 pin PSUs work with 24 pin MOTHERBOARDS?. Did they put an adapter in your computer?I happened to read this just before. And no they did not put an adapter in. This is basically what it looks like,
http://www.hardwareanalysis.com/content/image/11254/

I have come to the conclusion that it is not my PSU that is preventing start up. After starting my computer for a second timeit stays very cool but persists to keep reseting. Im not sure what it could be :'(maybe its something to do with my motherboard setting being changed from my hadrive.just to report back to you,i just booted it up with the windwos xp disc and it seems to be working fine. Though the gpu is getting a little hotter than usual.What's your understanding of the 24-pin motherboards? Do you know why the 24-pin connectors were introduced?

I believe the extra 4 pins were put there to power certain PCI Express video cards. You have a PCI Express video card, right? If you believe you do not need a 24 pin PSU, then let's have an answer to the basic question. Why does your motherboard have a 24 pin connector? And, what about your current configuration makes the 20 pin PSU adequate vs. a 24 pin PSU? And, if you don't need a 24 pin PSU now, what change or upgrade would require getting a 24 pin PSU?Why don't you reinstall Windows with the current hardware? Do you have a real Windows CD?

If they fit a new mobo and it doesn't work, I think that would warrant a return visit.