Open the RUN box and type MSCONFIG. Look on the STARTUP tab. Everything that is checked is starting with Windows, and a good portion of those are unnecessary. If you uncheck something that prevents Windows from starting, you can boot into safe mode and put it back. Read what's starting and use logical deduction, and uncheck stuff until the problem goes away.
Go to
http://housecall.antivirus.com and do an online virus scan.