The COMPUTER that you are on now, does it have an optical drive that can burn a disk?Indeed it can Quantos.According to the manual: http://www.notebook-manuals.com/laptop-manuals/service_manual_for_SONY_VaioPCG_PCG-R600_991327d635_R600MF_H_UK.pdf, it doesn't look, it has recovery partition, but it supposed to came with recovery CD.
Just in case, you can try pressing F10 key at Sony logo.After digging around I managed to find the bits and pieces associated with this laptop. Predictably, the recovery CD is not there. I found the XP and Sony manuals, loads of useless Microsoft Office bull and all the packaging. No recovery CD.

ALSO predictably, tapping F10 at startup just sent me to a black screen that did nothing for 15 minutes. Sounded like the laptop had just given up, wasn't making a sound and no lights were flashing.

I took a look online to find a recovery CD for the Sony Vaio PCG-631M, but couldn't find anything that looked really helpful.

http://www.user-guides.co.uk/Sony/Recovery/Sony-Vaio-Recovery-CD.htm
has some recovery CDs for sale, but I don't much fancy buying one for a terribly old laptop that I only really want so that I can work outside of the house. Also, doesn't look like my exact model is listed there. [Apologies if this breaks any linking rules].

Thanks again, any more thoughts?Borrow an XP CD and do a Repair Install of XP...

The version and Service Packs have to match...
Read THRU it to GET familiar with the process and print out a copy for reference.

Done properly all you apps and data will be intact...
Done improperly your data will be gone but you will have a fresh XP install.

Best of Luck.Success

The data on the drive was never important to me, but I was just looking for an easier/quicker option than a complete reinstall with a borrowed XP disc. However, I managed to get a hold of one and now the laptop works fine! Had a few issues with display drivers after installation because I have none of the discs, but now I'm in and can check all the specs, everything is much easier!

Many thanks patio, Broni and Quantos!Nice job