Well, #1 really is true, but if you suspect something is in your machine, then you still want to check! I totally understand, and there are some free options. Having used OS X since it first came out, my gut tells me it's most likely something else, but certainly no harm in checking.
I'd start with ClamXAV. It has a Mac graphical front-end, along with the unix back-end of the popular clamav program. You can get it here for free: http://www.clamxav.com/
Before I'd even bother running it, though, go into Applications->Utilities and fire up Disk Utility. First Verify the drive. If it finds problems, you'll need to reboot from another disk to repair them. (For what it's worth, the last time I had a Mac that started getting really sluggish, the Hitachi hard drive I put in was failing; replacing it solved the problem. So make sure you have a backup of your important stuff!)
If Disk Utility gives you the green light that no repairs are needed, then next run the Repair Permissions command; that often can solve slowdowns, as the machine will really drag trying to open a file that it should
be able to work on, but can't due to a permissions misconfiguration. (It's not something you did; it's usually badly-written Installer programs that cause this.)
If all that checks out OK, then go ahead and run ClamXAV and see if it finds anything.
If it doesn't, the next troubleshooting step I'd try is creating a new user account, logging out of your husband's account, and logging into the new one. (Even the Guest account will work fine for this purpose.) Does that fix the jerkiness? If so, then there's something specific to husband's user account. Maybe some software that's set to auto-launch that's dragging things down?
Trying the system with a fresh, clean user account will do half the work, because you'll be able to figure out if it's something in the OS or hardware (which will carry over to the clean account), or something specific to his account (which will go away on the clean account).
Hope that helps.