My 2 cents:

If you suspect a virus, do these 3 steps:

- Run a MalwareBytes quick scan
- Run the ESET Online Scan
- Download the Emsisoft Emergency Kit to a USB and run the 'deep' scan.

That should get rid of almost all the malware. If you have a rootkit, it will become more difficult.

Also, run the Secunia Online System Inspector so that your system is up to date and safe. Then also delete unwanted, outdated or unused programms.
Chrome or Firefox are also faster than IE.

For your info, there are a lot 'Hijack This' forums online where experts give you advice and programms with what to do. You post the logs and they provide the steps you'll have to take. Almost every time, the result is a clean, safer and faster computer. Might be worth looking at!
The MBAM forum, I heard good things of.

A virus can be the cause, but there are a lot of other factors that slow your PC down. So a HijackThis is a very good option. Then you are sure everything is ok, from the updates to antivirus to your harddrives health.