First step after system restore

First:

Please know I am practically computer illiterate.
I am not positive WHAT I have done to my computer.
I am completely grateful for whatever time/help anyone can give.

My computer is an ancient HP (2003) with Windows XP. I use it mainly for Photoshop and tame Internet viewing.

My memory is splotchy on the following: Yesterday I got a black screen telling me system32/config (and some other things) were no longer accessible. Did my usual novice attempts at fixing things (turning on/off, checking connections, pushing random buttons). I thought I remembered F2 did something, so I pushed that which led me to a blue screen and I think I eventually ended up pushing F10, though I can't be sure if I followed through on where that led me or if I cancelled and kept looking till I came to another black screen which gave me 3 seconds to choose one of two options - maybe either restart or restore? Anyway, I followed through on one of the two screens. I think it was system restore, it took about 10 minutes. It said data files would not be lost and applications would be restored to what was there at the beginning, applications and updates I installed would be lost.

Now to my question - what do I do first? Do I need to update xp first? How do I download antivirus w/o getting infected to begin with? I already did one stupid thing and got online w/out being protected to download/run norton removal tool. Probably got infected being online just for that.

Any pointers greatly appreciated!

Amy