This is going to be quite long, so I've underlined the important parts.

I was getting BSODs while booting, working or even shutting down. Each one said something different.

A couple mentioned these:
PFN_LIST_CORRUPT
IRQL_NOT_LESS_OR_EQUAL

Another one was telling me to disable AV, backup software and defrag utilities.
During boot, I got a couple BSODs mentioning NTFS.sys and USBPORT.sys

I bet I've missed another two.

Of course this was driving me crazy - especially that there was no particular pattern and I was bombarded with BSODs. I had no newly installed hardware of software, so I attempted to format and re-install Windows. I even kept on getting different BSODs telling me all sorts of BS like

make sure I have enough space
boot into safe mode and check disk
your BIOS doesn't support bla bla bla, so go and update it

I did go through the process before and I'm sure I never got such instrucions.

Thanks to this, I couldn't update my site, I got stuck at 30% in NFSU2 and my Mom went mad at me 'cause she has to do some research for university.

Then the problem got worse. When I turned it on, my PC would keep rebooting like there was no end to it (I mean Windows was not loading, the PC kept rebooting continuously on its own).

I was extremely pissed off and waited for pro assistance to arrive. After all this, for some weird reason I decided to try again. Miraculously, I was able to successfully format and install XP. Naturally, I was very happy and it worked for a whole day without any problems. I almost thought the problems were gone for good, but last night, while surfing the net, my modem's driver interface crashed, so I had to reboot. But Windows won't start! More BSODs. I was so angry now I didn't even try to fix it.

Later when I calmed down, I came back to see what I could do about it. Surprise surprise! It worked fine. I was speechless. Since then I haven't seen any more BSODs, but there's another problem. Occasionally, my PC would restart on its own as if I hit the restart button on the case.

If you've made it up to here, I really commend you my friend. My question: what do you think was causing the BSODs and why does it now keep rebooting from time to time?