I'm trying to fix my kids PC its a bit old but was working OK,then it began going strange...

first it wouldn't find my monitor on start up no signal...
this still happens OCCASIONALLY but i just reset the bios buy unplugging the battery this seems to work..(temp)
but then when i even try to go into bios screen after 15 seconds it will freeze,screen,keyboard mouse,drives,Cd's all stop working and i have to reset it.
i have tried ram as i have spare yes the correct type and the PC says its OK on start,windows will attempt to load but 15 secs in freeze all STOP again.

all fans spin and you can hear the hard drive no bad sound off the board i.e bleeps to say there a problem please help....cheers Have you replaced that CMOS battery ? ?
This sounds like a PSU issue...borrow a known working one of the same wattage or greater and swap it in there for a few days to test.thanks for a reply but i have fixed the problem as follows...

1> replaced the Cmos battery with new (after 10 Min's out)
2. found that the glue holding the CPU fan had melted so it had slipped (only by cm) so replaced glue fan etc
3. replaced ribbon (again) damaged to hard driver's
4. reinstalled windows xp pro

this solved all problems but thank you again for replying hope it helps others

p.s know anything about windows 7 as i got another PC.higher spec worked great on vista with my At 1650 pro graphics card but has lighting probs on windows 7 , etc...
What exactly do you mean by lighting problems on Win7 ? ?
I'm RUNNING both the 32 and 64 bit versions...im runnig 32 bit as i only have 2 gig ram 4 gig required for 64 etc...lighting as in on games if i LOOK at a wall the coulors are BLOTCHY and square not in sync as in  well lightd areas even on old games the blotches appear or dark tunnels .ie in alone in the dark worked brill,but bad graphics in windows 7,because of lighting....Which ATI drivers do you have installed under Win7 ? ?SORRY been away...well the drivers i have are the only ones it will let me install via the ati/amd site the one off my disc came with card version 7.11 or the vista version 8.12 only hd cards are listed on the ati site 2400 and above, but the catalyst version will update to 9.3 but not my drivers says fail....any help to you...Use the drivers that shipped with the card....
I have an 8600 GTX eVGA and they work fine on both ver. of Win7...