Help please bevor I throw that thing out of my window

I try it again. well i'm new. i did what it says in that new things with delete old stuff and so on. but.... again. pc out after power went off, it startet new like in safe mode, task on the bottom is gray. suddenly no sound, no printer installed... norton found later: error regsvr32%windir%\system32\msxml3.dll. i did want it says but in RUN, it can't find that file. then i tried what it says on major geeks thing for newbies, but i even can delete old stuff (just some), it says windows installer service could not be accessed. this can occur if you're running in safe mode, or if the windows installer is not correctly installed. ok? well we moved recently from germany. usually my brothers (computer freaks) did everything, but i need really help here. so pleaaaaaaaaaseeee help:cry
by the way, we had a few weeks a trojan, problem solved, but just that you know. and i have xp, dimension 3000, in case you need this