This is, and it is an issue that MOST of us are experiencing, INCLUDING myself, as I have experienced this problem as well, and I, too am running Windows XP Service Pack 3 on a Dell Precision M6400 with an Intel Dual-Core processor.
Current specs:
Dell Precision M6400 Mobile Workstation
OS: Windows XP Professional, Service Pack 3 (version 2002)
RAM: 2 GB
Processor: Intel Pro-Duo Core P8600 @ 2.40 GHz, 1.50 GHz
Video/Graphics Card: NVIDIA Quadro
My laptop also has TWO internal hard drives, 500 GB for the OS and programs, and a 2 TB for storage.
When I received the update, I, TOO experienced this issue. -I thinking it was my resolution settings, (as I am comfortable with 800x600 as I am legally blind) I changed it to 1280x720 to no avail.
I've tried the repair function, AND ran AvastClear in SAFE MODE (as that is the recommendation, according to Avast Software's site) and I STILL experienced the error in question.
The PROBLEM isn't, or at least SHOULDN'T have anything to do with limitations, put by an operating system, but the program's SERVICES RUNNING IN THE BACKGROUND.
NORMALLY, this would call for a ROLLBACK, but that requires having an OFFLINE INSTALLER on deck to perform the rollback, and not many have access an offline installer. In fact, I had to LOOK AROUND for the last release in order to install it with my WiFi settings disabled, in order to change some settings BEFORE updating my virus definitions, while KEEPING my copy of Avast 12.3.2280 without getting updated to the current build.
I am sure Avast's developing team is aware of the problem, but we've yet to find a proper solution, that has so many users wailing in despair over their Graphical UI not performing properly.