And someone is here, (seriously, why nobody answer this?)
Looking at your thread, atleast you got the computer to run, thats good, else something might be horribly broken. If it runs, its propably mostly fine, just some software issues.
Since you mentioned WinXP, im pretty sure the main reason, your PC is low, is because of that, Im not entierly sure, but im quite confindent, that very few people run GTX 660 on a winXP platform, heres why:
You need Service Pack 3 to run that stuff (and service pack 3 makes stuff really slow), and its not really a supported OS anymore, move to win 7 or 8, (8 is what i would go with, a lot of old computers work well with it because of the low Hardware requierments)
Also the reason you wont get into safemode, is because the card is propably not supported there, and therefore it wont be able to launch in it, because safemode uses the bare essentials, it wont load crap, what aint needed, so you are stuck in minimal 800x600 or something, with no plugins/support for new hardware, it simply does not have the files, to cope with your new GPU.
SO, install windows 7 or 8, post what happens. IF some anomalyes occour, report them.
Not to rule this out, the chances are very low of this being true, but the motherboard might also be damaged, but hopefully not, because in most cases, it wont even get you to Windows screen, if that happens.