Did you try a Repair?
Control Panel -> Add/Remove Programs (or "Programs and Features" if you're on Vista/7) -> avast! -> Repair.
No, I hadn't. However, after your reply, I logged in as an Admin and did so. After a reboot, I was able to update the VPS. When I then went to Avast's Summary, it had an update button by "Program version: 5.0.393". I clicked on it and the program updated to v5.0.394, and asked for a restart. I allowed it and everything appears fine now. The VPS is at 100127-0, where I started this morning before installing the prerelease 5.0.393.

Hopefully, there'll be smooth sailing again. This was the first problem I've had with Avast, and to have it resolved so quickly pleases me no end.

@ Gopher John
You mention "I uninstalled V5 and ran the new AswClear.exe," but the version number of the one on the http://www.avast.com/eng/avast-uninstall-utility.html page is the same version as that used with 4.8 I believe.
So I don't know if it has been changed/updated the version number is 1.0.0.1 although the file properties show a date modified of 31 December 2009, 12:41:03, which pre-dates the release of avast 5.0.377
I downloaded from
http://public.avast.com/~forejt/aswClear.exe yesterday as it was posted to the forum. The version number hasn't changed.
Internally, it shows created on January 26, 2010
File: D:\Downloads\John\aswClear.exe
Size: 157232 bytes
File Version: 1, 0, 0, 1
Modified: Tuesday, January 26, 2010, 11:07:55 AM
MD5: F7B9DECDEDD836F080F98704065BDAD7
SHA1: B7B939CAFCCFB052FB3BD8A9DB92701FCC87010B
CRC32: 99A9A7D4
The earlier version that I downloaded from the old link. Internally, it shows created on June 05, 2008
File: D:\Downloads\John\Security CD\AntiVirus\aswclear.exe
Size: 230776 bytes
File Version: 1, 0, 0, 1
Modified: Sunday, January 24, 2010, 8:55:47 AM
MD5: D8405231FC86373340A38974BF216743
SHA1: 6106CE11BB519097A699C6BC5F47624712477A65
CRC32: 89085BAF