If it does open in Safe Mode, what have I accomplished toward getting it to open normally?"
You have demonstrated that if it works in Safe Mode but not Normal Mode that some (yet unknown) process is the cause of the problem.
You can then investigate, in Normal Mode, a bit further by isolating other possible causes by disabling selected processes to see if the problem corrects itself.
310560 - How to Troubleshoot By Using the Msconfig Utility in Windows XP (or Vista)
You should also check in Normal Mode with a different user logon to eliminate any Profile problems as being the cause.
Checking for malware and the like is an idea too.
Some general things to try.
See if System Restore will get you back to a restore point before your problem with Explorer.
Try running ChkDsk to check your drive for errors. Right-click your Drive icon/ Properties/ Tools/ Error Checking. Select both boxes.
Run the System File Checker program from the Run Box by typing.....Sfc /Scannow in it and have your XP CD handy.
HOW TO: Verify Unsigned Device Drivers in Windows XP
If they don't work you could try repairing windows by running it over itself. You will lose all your windows updates but your files will be untouched.
How to Perform an In-Place Upgrade (Reinstallation) of Windows XP (Q315341)