Hello fellow Geeks :) Im in need of help!

Hello there. Im new to this so bear with me please.

Im running windows xp 34bit completely stock pc.

Recently ive noticed that my F-Secure doesnt load up when i want it too. It loads up and prevents viruses (it deleted 3 recently) and does scans for me, but apart from that, it will not go to the 'main screen'.

Secondly, my office doesnt work properally. My word, excell, powerpoint, assess ect works. But every now and then, they dont, and ask for my microsoft office 2003 key. (Which has been missing for about 5 years). Tried downloading a cdkey finder (tells you all your keys on your pc, so i can type it back in) but that failed. Now i have a windows automatic update getting ready which fails every time. Its called
'Installing office xp service pack 3 (update 1 of 1)... '
This then fails half way through asking for my 'microsoft office xp professional with frontpage' cd-rom (which i dont have). Then the update fails.

Thirdly, i recently installed a nvidea 9800gt graphics card and i believe i may have plugged a wire from that into the motherboard wrongly. Lots of bluescreens asking about the card.

Lastly, my pc is running mega-slow. Crashing almost 3 times every half a day, freezing alot, and bluescreening 2/3 times a week. Today i have run 3 virus-checks with f-secure,SUPERanti-spyare and HijackThis(which i didnt understand one bit.) 1st 2 saying its clean. I defragged the whole pc - still slow and crashing. Deleted all unwanted programs and i have over 120+GB free space still. - Still slow and crashing.

I am completly stumped, any help would be much appreciated.
Thanks in advance,

Kane Leeming