After using my computer 1 or 2 hours it hangs and icons vanish

Hello,
sir,after using 2 or 3 hr my computer gets hanged and in main menu my icons are vanished so blue screen appears .so i forcefully shut down it and agian start my windows its starts fine but after some hour same things hapenned.pls sir solve this problem i will be grate ful to u.

system information-processor-pentium(R)dual core cpu E5700@3.00GHZ ,RAM-2.00gb ,system type-32 bit operating system pls described me step wise bcoz im just a new learner