Creeping Slowness

If you can answer "yes" to any of the following questions, your computer might have a virus.

Is your computer running very slowly? A common symptom of a virus is much slower than normal computer performance. However, there can be other reasons for slow performance, including a hard disk that needs defragmenting, a computer that needs more memory (RAM), or the existence of spyware or adware. For more information about spyware, see How to tell if your computer is infected with spyware.

Are you getting unexpected messages, or are programs starting automatically? Some viruses can cause damage to Windows or some of your programs. The results of this damage might include messages appearing unexpectedly, programs starting or closing automatically, or Windows shutting down suddenly.

Is your modem/internet traffic or hard disk working overtime? An e‑mail virus works by sending many copies of itself by e‑mail. One indicator of this is that the activity light on your broadband or external modem is constantly lit; another is the sound of your computer's hard disk continually working. These are not always symptoms of a computer virus, but when combined with other problems, can indicate a virus infection.
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If you are up to date with your virus scan files and if you have scanned using several web based scanners and they show negative result the high probability of the slowness being virus is very small unless coupled with other instability symptoms.

At that point looking at the other slowness causing issues is appropriate.

An honest answer is... you will not be 100% certain unless you reformat the drive and reinstall the operating system, which is a radical last resort and not advisable unless performance is so bad the device is virtually unusable--even so, all other avenues should be explored, sometimes the issue has terrible effect yet quite simple remedy.