I have recently added a UPS (Belkin regulator pro 525) to my system and installed the Belkin Bulldog software with no problems at all.
There is a UPS monitoring (service) facility in XP mine is at Manual and is Stopped, with no ill effects. I have seen that some UPS require the service (I think some older APC ones), but if yours has been working ok previously I wouldn't think that is a factor.
The only thing avast could have to do with anything is if the UPS software was thought to contain a virus (which you haven't reported), but nothing else.
I did have a similar error no battery backup present,but that was when I first added the UPS and before installing and configuring the software.
I assume you are not getting any warning lights on UPS power up test?
You could try the old uninstall reboot, re-install of the UPS software and see if that helps.