Ok! I think I did it.
To be honest, I'm not exactly sure how it worked, but I'll list the steps I took.
I downloaded the trialware version of Symantec Encryption Desktop Corporate (https://www4.symantec.com/Vrt/offer?a_id=109355), which gave me the WDE tools mentioned above.
I also downloaded the latest version of PE Builder (v. 3.1.1.0a) from http://www.nu2.nu/pebuilder/#download.
I built a BartPE boot disk and installed the WDE drivers using pgppe.exe from the Windows PE & BartPE Tools for Symantec Encryption Desktop 10.3.1 instructions (http://www.symantec.com/business/support/index?page=content&id=TECH210436) as mentioned above. The pgppe.exe must be run from command prompt. You must be in the directory where pgppe.exe resides when you run the command. The command in the pgppe.exe program is "/winpe". The next field is the location of the BartPE folder you create. The last field is the location of the WDE files (in my case, from the trialware version).
There's no obvious confirmation that the drivers are installed successfully (at least I didn't see it). I believe I installed the drivers correctly because command prompt flashed some new text quickly and then closed. Even though I'm pretty sure the drivers were installed, the first BartPE disk I made didn't work. My mistake was that I had also included the Symantec PGP software files in a folder (the /WDE folder). So I made another BartPE without the folder.
I downloaded a program to turn a folder into an .iso file. I used Folder2ISO, and I downloaded it from Softpedia http://www.softpedia.com/get/CD-DVD-Tools/CD-DVD-Images-Utils/Folder2ISO.shtml
I made an .iso file from the BartPE folder and burned the .iso image onto a CD.
I popped the CD in, and the virus screen came up again. I didn't think it was working, but I restarted and tried again just to make sure the CD was booting first. Computer started up normally...the background was mine, but nothing else popped up. No icons, no start bar, no virus screen. Nothing but the background. I was expecting the BartPE to bring up a different screen. How could it access my background if it's a preinstallment environment?
I used ctr-alt-del to bring up task manager, and from task manager I clicked "File" and then "New Task (Run)". I ran "cmd" to bring up command prompt.
I used the system restore command (for XP it is C:\windows\system32\restore\rstrui.exe). I chose an earlier restore point and restarted.
On start up, I got a message that said no changes had been made. It seemed like it didn't mean "no recent files have been deleted" but rather that the restore was unsuccessful. So I tried another restore point. Same error message. So I took a chance and let it start up. No virus screen. I downloaded Malwarebytes and scanned. Found 52 bad objects. I couldn't confirm if any of them were the virus, but my computer is working. I also ran Microsoft Forefront Client Security, Spybot, and Adaware, and found no other virus-type threats. So I think I'm ok.
Mine was a rare case, but I hope it helps someone!
dcats, thank you for your help! I think the next step is to decrypt the harddrive so I don't have this problem ever again.