Malware suspected! Yahoo IM is being hijacked

HELP!!! My Yahoo Instant Messenger seems to have been hijacked or infected with a testy little virus... When I launch Yahoo IM it seems to work fine, until it logs me out saying: "yahoo messenger has logged out automatically because you have signed in at another location or device." The only problem is that I haven't logged out or logged in from another device. When I try to relog in I get into a 'tug-of-war' with this 'other' entity trying to control my messenger. I have run multiple malware/spybot/virus hunting applications and cleaned my machine to the best of my knowledge...I've also tried to change my password for my Yahoo mail (as most of Yahoo's web based applications are tied to that username and password...) That seems to fix the problem for a bit, until I get the same message saying I've been logged out from messenger and my new password is no longer active...it's only with my 'old' password that I'm able to reaccess my yahoo services again...
To add to the problem. I've been running this Yahoo IM on two different PC's, so not sure if one or both PC's have been infected with malware and both require thorough cleaning. Both PC's are running XP Home with SP2 installed. My Yahoo IM is Version 7.5 (it's uninstalled now). I'm at a loss, and would appreciate help anyone can offer...Please advise...