On downloading CIS it only gives me the option to download the XP pro SP2 version of CIS and so I was just wondering whether its fully compatible with XP pro SP3 before I install it.
Hello!
Yes, CIS is fully compatible with SP3. 
Thank you. I previously have avira antivir as my anti virus (with comodo firewall), but my computer was behaving rather strange, so I have now installed CIS. It only found one problem on the initial scan, and is now scanning after updating. Fingers crossed that this sorts out the problems.
Hmm, interesting you should mention the strange behavior Alexei23. I am currently using the same setup you previously had (CIS Firewall and Avira AV), and I too am having some strange behaviors. For example, on this bootup, my taskbar is gray, not blue. Some startup programs won’t load at all, others load but don’t display a tray icon. Sometimes I will double-click a program icon and nothing happens at all. Then if I open Task Manager, it shows multiple instances of the unresponsive program running. I am going to make a new post for help on some of these issues, but I just wanted to mention that this strange behavior has only ever occurred with the above firewall/AV combo. I’m sure some of it is me just not knowing what the heck I’m doing yet! :THNK
Kenny