heres what im experienceing...........and if you have any idea what this may be please dont be shy

i have obviously picked up something ....i was trying new antivirus programs and downloaded avast...now avast picked something up and couldnt issolate it and move into the "vault" i uninstalled avast and now heres the tricky thing.

i CANNOT download ANY antivirus programs..i have tried mcaffe, norton, pc cillin, panda , even went to downloads.com and tried the new stuff............it seems to stop all downloads of any antivirus., ( i can still download anything ..but antivirus programs) and i mean i can download anything.....but when i try to download antivirus programs they freeze stop and a timeout error is presented....
now the logical thing is to go to trendmicro and get an online virus scan ( housecall)...been there 5 times and when downloading the required plug in................cancelled (timeout error) i actually got it to 78% then stopped

heres the big problem............im running xp home ........and this thing wont let me do a system restore.........i have gone as far as a month back and still receive a "system restore incomplete" error................

i have exhausted my resources and am now asking if anyone has heard of any nastys that infect a system like the one described above.?

a link would be great ., as i dont mind doing reasearch...if i know what im looking for
thank you...and if you have any further questions i will do my best to answer ..thank you again
~frustrated~