It sounds like you could have a virus or other malware. Get your antivirus program up to date and boot into Safe Mode. Note that some viruses can hide themselves from your antivirus program in normal mode, so you really need to scan in Safe Mode. To
get into Safe Mode when you first power up, hit F8 about once a second until you get the menu and select Safe Mode. Then run a full system scan.
Microsoft has suggestions and offerings at http://windows.microsoft.com/en-US/windows7/how-do-I-remove-a-computer-virus
Forum moderator Keith has some suggestions along this line at http://answers.microsoft.com/en-us/windows/forum/windows_7-performance/windows-explorer-has-stopped-working/6ab02526-5071-4dcc-895f-d90202bad8b3
If that fixes it, fine. If not, use System Restore to go back to a date prior to the beginning of the problem. To run System Restore, click Start -> Programs -> Accessories -> System Tools -> System Restore. Click the box that says Show more restore points.
You can check for corrupted system files. Open an administrator command prompt and run SFC if the above doesn't help. Click START, then type sfc in the search box, right-click SFC.EXE and click Run as administrator. Then from the command prompt type sfc /scannow.
Finally if all else fails, you can check the rather cryptic system event log. To do so click Start -> Control Panel -> Administrative Tools -> Event Viewer. Once in Event Viewer click on the system log and scroll through the entries looking for those flagged
"error" to see if you can find hints as to where the problem could be.
I hope this helps. Good luck.