Another problem might be because your other old files my be filling up with junk from the web. Do a file search for 'temporary internet', 'cache', or 'post' files. You may find literally hundreds of useless files clogging your hard drive. Just delete all the junk and run the disk defragmenter (Go to My Computer and click 'Help' and then enter 'defragmenter'.)
I routinely defrag and clear the cache.
I also use SpyCop and Evidence Terminator (the latter is supposed to delete all useless files, like logs and .bak and lots of others. It claims to speed the computer.)
So far the only "spy" program SpyCop found was Support.com, which came with the computer and is supposed to be there (the Support.com website says so).