a disk read error occured press CTRL+ALT+DEL to restart

I started my computer this morning and everything seemed ok until it suddenly started ending programmes and my taskbar would dissapear then reappear and continue to do this until i restarted. When it restarted it wouldn`t even start windows and came up with the error "a disk read error occured press CTRL+ALT+DEL to restart". this was after it run through the disk check - all my disks appear to be working.

I installed a new 1GB RAm stick the other day and it seemed to work fine although i didn`t notice a huge increase in performance. I also installed a new Sapphire 1600XT card several days later which seemed to work ok although my computer restarted itself when playing Tetris (i know, i am very sad!)

For your info my system is as follows:

Gigabyte MOBO (not sure of model)
Intel 6300 Dual Core 2
1GB of DDR2 RAM (Kingston)
2x512MB of RAM (unsure of brand)
1 SATA hard drive
1 IDE hard drive and IDE DVD-ROM Drive on the same IDE cable
Sapphire 1800XT Graphics card with the latest drivers

Any assistance would be much appreicated!

Thanks.