It's true that a slow computer could be a sign of a virus or other malicious software, but it could be something else.
If your computer is acting funny, take these three steps to help diagnose the problem:
1. Scan your computer for viruses with Windows Live Safety Scanner.
2. Remove viruses with the Microsoft Malicious Software Removal tool.
3. Download antivirus and antispyware software that will update automatically. Microsoft Security Essentials is free and works with Windows 7, Windows Vista, and Windows XP.
If your computer is still slow, you could use the free tools in Windows to free up disk space, speed up access to data, repair disk errors, and more.
To learn how, see 5 ways to speed up your PC.