After using RegVac, on the next reboot I get a "unable to log you on because of an account restriction" message and cannot log in as admin as usual (I have no other account), though I have never ever used a password to log on, and am used to simply turning on the machine and getting the Desktop.

This has happened off and on for a couple weeks, and I have to use the safe mode to roll back the registry with WinRescue XP.  Fortunately, safe mode does not demand any password.

RegVac is not alone in causing this problem, but some program installers do a similar job the the registry requiring the roll back in order to log in again.

I would like to use RegVac, as I have hundreds of useless registry entries, but cannot for now.  And, yes, I have the latest RegVac version.

What do I need to do to protect myself against this problem (not just with RegVac but some installers, too)?

   I don't think that RegVac is doing that.  I have never had RegVac do that, I have never heard of anyone having that problem with RegVac, and I don't even see how RegVac could do that unless you are running Registry Pack or Registry Restore (and you have Norton).

It is not Regvac but your dangerous practice of not setting a password for yourself. You should always have a password.
If you want to log on with out keys strokes get a finger print reader or a free utility like TweakUI or Safe Logon, give yourself a pasword and then set the system to logon on from local boot with autolongon i.e without having to enter the password.

What is dangerous about not having a password?  My computer is in a locked home with no children or anybody else that might mess with the computer?  I have had PCs over 18 years and never felt the need to secure my home computer.  Of course, it is possible to break into a locked home and steal a computer, but then what good is a password then?

By the way, it is TRIVIAL to reboot a computer with a CD and then change or remove the password, so a password is only relevant for keeping out a person who has just a few minutes of unattended access to your computer or a small child who does not know how to use a boot CD yet.

For Ray:

Yes, indeed RegVac CAN and DID repeatedly mess up my log in, requiring me to restore the old regs!  But I "cured" the problem by setting a trivial password with a hint so obvious that anybody could log in if asked to, and the problem has vanished!  And TweekUI has been set so as to log me in automatically, and I get the Desktop without loggin automatically again, despite the weak password.

For Ray
2Shlow  has too narrow a view of the purpose of passwords, Windows XP is certainly  designed to be used with User passwords. Iif a PC is in a room where anyone can use it without any monitoring an experienced user could wander in and can log on even is there was password protection but it would take a litle time and leave traces. But though a locked room will protect such a PC from casual strangers if the computer is connected by broadband it is exposed to attack by remote users. Among my clients businesses with many users  passwords are always  part of their security plan.
So a pasword has three main uses,
1 -Separate access  for a varitey of users on a single work station
2- Protection against remote users while browsing the internet while logged in as an administrator,which is what so many users do,.
3- Ensuring that Windows logs in correctly. It is a technical fact that Windows XP will often fail to load all task-bar icons correctly and connect to the net slower if it loads strasight through without a password, unless the Universal Plug and Play service is disabled to reinforce security but this brings problems if the computer is part of a network or  using a router for Internet access; HOWEVER if 2Shlow is just using his PC in a safe room, disabling UP&P service could solve his  Regvac problem. So I do not think RegvaC is at fault in his case.
I have been in the IT business for over thirty years and my experience is that passwords are no trouble to use and are funtionally useful.

If it is true that "disabling UP&P service could solve his  Regvac problem", then since UP&P has been ALWAYS disabled on my system, the problem must never have happened.  But in fact, it did happen!

By the way, my computer is behind a router with a password (so the system cannot be snooped into from outside) and a password to boot the computer will certainly not save a system that is open to the Web from outside due to lack of a router or a router with a default password.

I see no point in having a password to boot, other than to allow RegVac to work.

Well then you have an easy option

1) Use free  Tweakui to set your boot configuration (or buy Auto Safe logon) so that your machine boots with a password without you having to enter the password but it is still in force against remote users.

2) Stop Using Regvac.
Though you may have many useless registry entries they do no harm. It is best not to use registry cleaners ecept as a a last resort. If all is working well the
"If it ain't broke don't fix it"