There could be many reasons for this. Here are some things to try
1. Check the hard drive
Start/Run and type in
At the command prompt type
chkdsk /r (there is a space before /)
Now you need to reboot and chkdsk will run before boot up. This could take a while, so be prepared for that.
2. Defrag the drive
3. Clean out temp files with CCleaner
4. Run your antivirus and antispyware
5. Open the case and blow out all the dust and dirt with a can of compressed air.
See if any of these help :smile: