More often on the field we encounter unusual problems, one problem I just encounter a couple months ago is an ailing 40GB HDD.
Actually,the hard drive is working fine, no bad sector etc... only there are times that the hard disk can not be detected by the system BIOS and that you have to re-socket the IDE cable to be detected again by the system, sometime the partition get corrupted and that you have to re-format and re-load everything.
At first you have to suspect the IDE cable or being infected by a virus,but as the problem keeps coming back you have to start thinking that something wrong with the hard disk.
The strange thing here is that when you reformat the disk or check with a utility program, everything is fine. Only to encounter the problem after a month or so of operation.
Next time when you encounter similar problem such as this one, try replacing your existing hard disk with a different or new one.It will probably solve the problem.