The dreaded Blue Screen Of Death.
Here's a link to a Blue Screen Error code list. This site is a MUST have in your bookmark list:
If you ever need to check the BSOD code just take the last characters from the first block of 0x00000051.
Remember the 51, visit the above site usually from a another working system, if your is Blue Screening on each boot, and check your error.
Usually running what's called a Check Disc repair (chkdsk /r) in the command prompt from the Operating Disc should fix it, well 8/10 times it works for me.
If the check disc does work, log into your system backup your data and do a full OS reinstall and start from fresh.