Explanation as to why my computer keeps crashing

About a year ago I aquired a Fujitsu Lifebook E series laptop from my work. About a month later the power supply stopped working so I checked the power supply and it was fine. I then took apart my laptop knowing it was something internal. I fixed the problem and put it back together. It worked perfectly again. One day i was trying my password and (me being stupid) kept putting in the wrong password. It locked my account out and asked for an administrator to unlock it. This username wase only username available on the computer so I had to reinstall windows onto the computer. After that it worked fine for about a month before it started giving me problems. It started to randomly crash and I just ignored it at first but the crashes got worse. After a while a blue screen started popping up. Afterwords it would continually crash, corrupting files. When I tried to turn it on, it would do a disk check and delete corrupt filles from the c drive. Then if I left it alone for like a week and turned it back on, it would run for a little while and then go through the same process, I am starting to wonder if it is the hard drive or if it is the software on the computer. The errors on the computer are not always the same message either. Sorry this explaination is vague but it is the only one i can give you.
P.S. I do not know if me taking apart the computer had anything to do with it, i just thought i would include it.

1. Scan for viruses, etc.

2. Run ChkDsk.

3. Do a full harddrive diagnostics by utilizing the free utility your harddrive manufacturer will provide. Run both the S.M.A.R.T. and Long tests, especially the Long.

4. Then read this link and following the advice that applies to your situation.http://www.techspot.com/vb/topic51365.html