I've only tried the manual scan twice, but both times it said "unable to scan..." for each file. What's up with that?

It did find one virus during the on-access scan and I prompted Avast to delete it. So I know there was at least one virus on here. That's why I wanna do a thorough scan just in case there are any more. But I can't because the stupid thing won't scan anything except with the "live" scan thingamajigger.

you should try to scan in safe mode. there are tutorials that can help you if you don't know how to get into safe mode.

edit.
if that doesn't work, maybe you should try uninstalling and reinstalling the AV. also, there is a tutorial about that AV. however, i don't think you're doing anything wrong. just a glitch probably.

Scanning in safe mode didn't work. I guess I'll have to reinstall.

Try doing a couple of the free online virus scanners:
Trend Micro Housecall <- Use "Autoclean" and manually delete what it can't clean.
Panda ActiveScan <- Accept default settings. (does not remove adware/spyware but will autoclean for viruses & worms...and scan for rootkits).
F-Secure Online Scanner. <- Be sure to follow the directions on the F-Secure page for proper Installation. (also checks for rootkits).
BitDefender Online Scanner. <- Add a check by "Autoclean".
eTrust Antivirus Web Scanner. <- "Cure" whatever is found. Be sure to read the eTrust Antivirus Scanner Help Guide before scanning.

I tried to get the activex controls for Panda Activescan but an Avast alert said it was a virus. I moved it to the chest.

These controls are legit, right? Should I have ignored the Avast warning?

Hello PCIlliterateGal

Panda Antivirus does not encrypt its virus definition files that is why avast detects them as "infected"
I suggest to you to try another online scan like Kaspersky or F-Secure Online Scanner.

You can also schedule boot time scan with avast!

Schedule Boot-Time Scan with avast! Click on the little button placed up in the left corner, and select Schedule Boot-Time Scan.


Next, choose

• Scan all local disks
• scan archive files
  
  
• click on Schedule

On the next dialog Operating system restart needed select Yes



Now avast! will restart your computer and start to scan before Windows fully loads. If detects infections while boot time scaning, you will be given choices for actions, choose move to chest actions and don't delete anything.

If this also fails then try the steps bellow:

To repair avast click Start > Settings > Control Panel > Add/Remove Programs scroll to avast! Antivirus and click Change/Remove button from the options choose Repair and click Next. You need to be connected to internet to repair avast!

If the above does not resolve the problem, try this

Download avast! uninstall utility do not use it yet.
Uninstall avast! from Add/Remove Programs, reboot.

Run aswClear.exe Reboot.

Now install avast! again!

Hope this will help you to resolve your avast! problems