O_O ..... geezzz Silver...
OK, well when upgrading I normally start with 1 thing at a time so if something goes wrong there is no doubt what did it. Takes longer but it helps if something does go wrong. I realize that is of little help to you now but none the less.
Now as far as your issues...POST errors vary depending on your motherboard/BIOS however 1 long beep and 2 short ones is normally a Video card error. Maybe it wasn't seated right (pushed all the way down) or maybe the card you got was bad, maybe there is a compatibility issue (say you have a PCIx card in a PCI slot or mis-matched voltage levels) there is also the possibility that when you installed the card that you weren't properly discharged and ESD (electrostatic discharge...i.e. a shock to the card) occurred nuking the card (seriously takes very little voltage to nuke a card). It is hard to say for sure but I would try the card in a different computer and see if it works there or not. Either way the card carries a standard warranty which will cover the replacement of the card if it does turn out the card is bad (I have bought a new card that was bad before so while it is rare...it does happen).
I assume that the RAM is fine since you didn't state that the POST errors occurred after you took the new card out and put the old one back in.
The message you get about the windows/system32/config/system being corrupt or missing is well.... really bad. It basically means your registry is corrupt. Look here for some details on things you can do: How to recover from a corrupted registry that prevents Windows XP from starting
I haven't read the article completely but it seems to cover all available options other than just re-installing all. As for what caused it...hardware issues have been known to screw up the data on a hard drive. I had a pc that I put a AGP card in what I thought was a AGP slot (it wasn't it was a AGP sized and colored riser board slot) and it whipped the HD clean. Maybe it was the card having issues that did it or make something else altogether.
The cause isn't much concern as just getting your PC back...try the options in the link and when you have that back then we can work on the card issue. If you can't get the data back and have to reinstall then check the compatibility of the new card to the motherboard and reinstall using it if you find the card is ok.
BTW...the below message means it was definitely a video card issue! Hammer wasn't crazy lol You could look at reinstalling the driver or updating it if there is a new version but normally it is the card going bad and causing the driver to not work right. I mean all a driver is a little piece of code that tells the operating system and the hardware how to talk to each other after all so it isn't far fetched.