what did you change since the computer was last working properly? Upgraded or installed any s/w or h/w?
Something to try is a system restore. To launch System restore, just type system restore in the start search area at the bottom of the start menu. Then choose a restore point at a time when it was known to run properly.
You wont lose any docs you have created since then but anything installed since then will be lost. Good idea to back up anything important anyway.
Could also be a memory or motherboard fault.