Usually this means your hard drive is filling up and there is good advice above.
If you look at your drive F properties and it appears you have more than double you RAM size left on the hard drive it could be another issue.

I've seen some malware that uses malware program to hide data on you computer. So a Spyware/virus scan couldn't hurt either.
Also this can occur from a failing hard drive. If sectors on the hard drive start failing, the data is automatically moved and that location is marked as bad by the hard drive and can no longer have data written to it. As this continues it starts to have less and less good sectors to write to.