Q: Imac 27" slow

No. It is not a virus.

Read the excellent page l;inked below to help with performance problems

http://www.thexlab.com/faqs/performance.html

You can upgrade after you fix the performance problem

Upgrade a computer with a problem is like doing brain surgery on yourself when you are drunk. Certain to only make matter worst.

Allan

See:

Mac Maintenance Quick Assist,
Mac OS X speed FAQ,
Speeding up Macs,
How to Speed up Macs,
Macintosh OS X Routine Maintenance,
Mac troubleshooting: What to do when your computer is too slow,
Essential Mac Maintenance: Get set up,
Essential Mac Maintenance: Rev up your routines,
Maintaining OS X, 
Five Mac maintenance myths and
Myths of required versus not required maintenance for Mac OS X for information.

It could be any number of things, but before you diagnose make sure you backup NOW and test the backup!

First check the RAM, it may be as simple as a you need to upgrade the RAM, please read this Apple Advice letter called Activity Monitor to read System Memory and determine how much RAM is being used.

If you have adequate amounts of RAM then next run Apple Hardware Test in extended mode at least 2-3 times back to back. If you get errors that confirms a hardware issue and a trip for repair is needed.

Yes, you need an external drive for backup.

Time Machine is a backup application that Apple introduced with OS X 10.5 Leopard.

Here is a link to an Apple tutorial on Time Machine

http://support.apple.com/kb/ht1427

Allan