yeah, if you can't go into control panel- user accounts and log in as admin I might supspect malware too.
ran a few scans recently?
fast user switching service enabled? i think that is the only service required to switch user profiles, start-run-services.msc (type that & hit enter) make sure the service is started.....
not sure if this would work but if you have or want to install spybot S & D in the advanced section you can uncheck startup items....