Win XP PC - a full restore?

My main Windows XP laptop PC is now around 6 years old. It has a small 32GB hard drive and my own files occupy around 8GB of this, yet the PC says the C drive has only 3.8GB spare. There is a small back-up partition (about 3GB) and the rest must be program files and Microsoft bloat accumulated over the years. I've already done defrags several times and deleted all the files I know are safe to delete. CCleaner is also regularly run to sort out registry issues and I have compressed files where possible. McAfree anti-virus seems to do a decent job of keeping out the malware.

Although I hope to invest in a new PC soon, I'm considering doing a full system restore back to original factory settings in the hope of recovering all the lost/wasted space.  A full restore will need a few days of work to reload updates (SP2/3 etc) but there must be a lot of unnecessary rubbish there which would be cleared by a full restore.

My plan is to use this "old" PC to run WSPR and some VLF programmes and use a new PC for everyday tasks.

Apart from the usual back-up of any photos and data that are important, does anyone have any advice before I go ahead?