My home PC has been getting critically short of space recently - medium-term I need to get another hard drive for it but I need to take the case off first to see if I have a SATA connection and it's just a hassle.
I deleted a load of old data last weekend but it hasn't made the dent I was hoping for. I noticed my C:\Windows\Installer folder was huge, like taking up nearly 30% of the used space. I had a look to see it was full of MSI and MSP files, which I didn't know for certain whether I could delete or not. However I Googled the folder path and discovered the existence of MsiZap, which will clean up old Installer files from C:\Windows\Installer. You have to install it and then run 'MsiZap G' from a command line. ZAP! 14Gb gone and a much healthier hard drive :-)
I also discovered Defraggler this week, which might be a better defragger then the Windows XP one or not, but even if it isn't it looks much better as you can see your data moving around the drive as it gets defragged.