Hello,
Install and run antivirus software, and a spyware/adware scanner.
There are many free and commercial antivirus software available. For example, Avast! Home Edition is a widely used free antivirus software, and McAfee VirusScan Plus is good a commercial antivirus software.
As for getting rid of spyware and adware, download and run
Spybot Search&Destroy. It's free and does the job. Make sure that you 'search for updates' in the program before performing a scan, to ensure that you've got the latest detection rules. Alternatively,
Lavasoft Ad-Aware Free Edition is a popular anti-spyware and adware program.