look Jelle, your way to deal with your computer won't convince anyone here but yourself. No firewall, no win/update, no service pack, and in normal times no AV either, this is suicidal. Your windows install should be ditched and redone from scratch with everything needed. I wouldn't use a system like yours even if I was paid for it.
Now I agree, but this is the XP exception (doesn't happen on Vista), that a service pack install can be a disaster. I've seen it happen with SP2 on XP. But there's a solution, and it's called slip-streaming >>> if it's still available there was a software to merge your current WinXP CD with SP2 and make a new install CD out of that. Worked like a charm for me.
Since I haven't used XP for ages, I don't know if SP3 has the same issue, but I doubt it. MS must have learned from their SP2 errors in the meantime (I assume you already got an XP/SP2 CD ...if not, I'm not sure if SP3 requires SP2 or SP1 already installed, see here:
http://www.winsupersite.com/faq/xp_sp3.asp ).
Once service packs installed,
there won't be as you said 9999 updates ... and windows updates don't slow down PCs unless your system is already broken somehow. This just doesn't exist.
Do you seriously think MS makes update for fun or just to make the users feel better or what
NO. These updates are there to fix security flaws as they happen and they are discovered,
AND USED BY MALWARE. So please drop the bs and understand that all this is not bloatware, all this is needed for stability and security reasons that you must acknowledge.
No firewall is mad, and tweaking a router's firewall (or did you turn this one off as well ?) won't do the job unless you're a qualified technician, which is not your case, and unless your router offers the needed tools, which is not always the case. So you do need a software firewall to control inbound and connections and avoid that your system might be used as a relay by web hackers, to control outbound connections to avoid trojans from connecting, all this exist and happen, browse this forum and others, and enjoy the feedback.
Every time I have checked computers from friends who didn't bother to protect their systems properly, I found malware, and OS slow downs because of the presence of malware. That's a reality. A computer is not a freaking TV that you can turn on and off and expect to keep running properly forever. It needs maintenance, on a regular basis, updates, scans etc...
Oh one last thing, use an AV in a permanent way too
If your system had been in use the way you described for two years, chances are it is infected with at least tens of spyware. Up to you if you don't want to change that. But then why come and ask for help here when you don't respect the most basic rules, doesn't make sense