it doesnt work... and it still doesnt work..

so, my friend asked me to help her with her modem - it just stopped connecting her to internet. she has winXP, all worked great for 2 years.
comp says that its an error, that some other device is using the modem-port.
(and she doesnt have any faxes or anything that could use it...)

well, first i thought it's some virus - well, after 3 antivirus programs, several ad-awares etc etc and they all state the complete healthness of the computer, i realized it must be something lese.
i deinstalled modem, and installed it again. repeated the process. nothing.
it says, though, that it isnt compatible with XP - but it worked before!
so..... what am i to do to help her?

thnx)