Messed up symbols when saving files?

Hi, I have a 2500jj and 90% of the time things are fine but every so often, if I am browsing files on the hard drive or (in the worst cases) saving files, all the text and folders are messed up, instead of letters it's just a load of meaningless symbols, so I cant browse folders, and usually the right amount of folders aren't there, just a moment ago there were only 2 folders in Root which were just symbols and I couldn't get into either of them. Has anyone else experienced this, or is my hard drive dodgy? I've had the same problems on and off for years so I don't usually blame the hard drive. Any ideas?

Thanks!