I would say do a complete FORMAT of the hard drive, this is where everything will be deleted of your hard drive and Windows ME will be installed once again. You will also have to reinstall all the programs that you wish to use.
If you do this make sure that you have all your personal files backedup as they too will be deleted in the format.
DELL - I'm not sure what you get with a DELL system. Recovery Discs, ME CD so could you tell us what Discs came with your system?
It sounds like after 2 and a bit years your HDD has become clogged with files and files and simply deleting everything off and putting everything back on will make your computer good as new.
The spec of the PC is fine.