|"Sfc/scannow didn't show any changes, but the virus may have been all over my harddrive."|
Sfc just replaces missing or damaged files Windows designates as essential system files, and it does not normally tell you what it did or did not do. It never hurts to run it, but I suggested running it to possibly replace a file ms update needs or depends on - it won't cure malware problems, other than it can replace missing or damaged files or infected system files the malware may have been responsible for.
When you get malware these days, you often get more than one thing at once. E.g. there is often a trojan involved that allows all sorts of malware to get through your defenses. CA may have found part of it but not all of it - that's common these days.
"If CA did get rid of it, the virus must have regenerated somewhere else."
It is more probable there was more malware there than that the CA software found, or the CA software fix was outdated and didn't fix something more recent that restores the malware automatically.
"The person that sold me this computer had wiped the drive and installed XP pro."
Did he give you a Windows CD, or a copy of one?
If he didn't....
Did the original Dell software installation have XP Pro? (You can probably look that up and find that in the specs for your model on the Dell site).
If it DID NOT have XP Pro, your copy of Windows is illegal, unless the installer used a Product Key for an XP CD he no longer uses. In that case it probably has an illegally generated Product Key.
If it DID have XP Pro, there is often an offical Microsoft sticker on the outside of the computer case with the Product Key on it. That Product Key can be used with any XP Pro CD, that is older than it having SP2c updates included, which is a very recent version, or a copy of such a CD, preferably on a CD-R. The copy has to be of the whole disk, using disk-at-once or simlilar in the burning software - copying the visible contents of the CD will result in the copy not being bootable.
If you do see that offical Microsoft sticker on the outside of the computer case with the Product Key on it, you can check whether the Product Key your Windows installation is using is the same as that.
E.g. search for Keyfinder on the web, by JellyBean whatever.
If it's the same, your Windows installation is legitimate.
If it isn't the same, you can use a utility to make it the same, but the key must be for the Windows version that came with the computer.
If your model is not more than about 5 years old, if you want to be able to install all the software that was in the original Dell software installation, which includes whatever Windows version it had, you could look on the Dell site to see if you can get a set of Recovery CDs, or similar, for your model. If not, try searching on the web.
E.g. I got such a set for a Compaq computer and it cost only 2x.xx, before shippng charges, and it has 6? CDs.