phone connection creating runaway cursor

[krup]

I do not know if it is a virus. The cursor started to runaway (putting spaces on its own) on internet web pages and putting cancel commands on its own on programs. It also did the same thing on excel worksheets. I reformated the c drive twice and reinstalled the original recovery disc. No problems until I tried to reinstall my internet connection. The second time no problem occured until I connected the phone line and immediately the runaway cursor showed up on the "pass word" line on the sign up page. It also started on an excel worksheet and stopped as soon as I pulled the phone connection it stopped. Is this a modem problem? Should I replace the modem?