I could not start my PC, a Dell Dimension E510. I got the message " Loading PBR for description 2 ...done

I think you will have to change the HDD !

try this do a total recovery or factory reset from windows recoverypartition.to get there tap on F8,F10 OR DEL.then there use the use tools there to recovery.do this when windows boot one tap a second.

If its an error message you received
If you want to know what this is the error code is related to

Click start control panel administrative tools computer management then select event viewer this may vary depending on what operating system you have extend the events there you will see errors in red you will no what exactly what the error was and at what time it occured Right click on the error message select properties you will see all details about the error or click start type in event viewer press ENTER you might be able work your way through the problem

Trying using a boot disk to see if your data is still there. I've seen this happen before and the virus had infected several of the files required for the computer to boot. When the virus scanner cleaned the files and the computer rebooted, the system files were gone.
What ever you do, do not reformat the hard drive or use a system restore disk until you are sure that you have no data that is recoverable on the drive.

That is the boot loader message to boot to the NTFS Primary boot partition (Primary Boot Record for Descriptor 1 is the Utility partition)
Normally, this message flashes and the system boots, since it is not there is a failure in the boot process.
#1 run diags on the Hard Drive (CTRL+ALT+D at boot) and see if it passes
If it does
#2 Boot to you XP CD, press R for recovery console, and run the COMMAND CHKDSK /R see if the system will boot
If no go, go back to recovery console, try the command BOOTCFG /Rebuild then see if the system will boot
If That fails, try reinstalling Windows
If Diags Fail or all the above fail, replace the hard drive

battery had all lights lit. Took it out and now the front battery display light is flashing. I have a new mother board could there be a problem with recognition?

I suspect that the viruses had infected some of the files that windows needed to boot up. The moment you ran the Mcafee, the scanning might have deleted or quarantined those files. That is the reason you're having this Loading PBR message because it is now trying to boot from the network and not from the hard drive. You will need to repair windows using the CDs that came with the computer (the Operating system CD/DVD) and run CHKDSK /R at the screen where it says 'repair your computer using the command prompt". If this does not work, chances are, you may need to reinstall windows and everything and that all information in your hard drive will be wiped out.

your hard drive has been corrupted due to improper shutdown

AH yes, Kristen very familiar with your issue, it is trying to load the primary boot record from a device it dosen't see , ( re: a floppy thats not there) you need to check the BIOS and make sure the booting sequence is correct (re:disable floopy if one dose not exist, etc) please write back if you need help doing all of these things

Boot to the Diagnistics FN+POWER at boot, run hard drive diagnostics. The PBR Descriptor 2 message is part of the normal boot process (directing the boot loader to Primary Boot Recond 2, the C: Drive) but usually flashes on an off so fast most people never notice it. Since the boot process is not going past the re-direct it is indicative of a bad hdd (Since the re-image process crashed)
Most likely you are looking at a hard drive replacement.
Regardless you are going to need an XP CD (you'll have to reinstall windows or run recovery console at a minimum)