Well, have an XP system too. Look to my sig below and you will see what I have.
Updating (upgrading) an existing avast! program does not always go as well as one hopes. Glitches come in where there were none before.
Best option I’ve found for an older system is to always do a clean install, especially when going up one version number. Example: avast! 5.0 to 6.0 or 6.0 to 7.0 or 7.0 to 8.0. It really is not an issue to re-register the newest version after a clean install and things tend to run a lot better when done this way on an older system. In-place upgrades within the same version number are usually not an issue. Example: 8.0.1489 to 8.0.1496 usually work out well, and so do not usually require a clean install be done.
If I let the XP system slide for a couple of days or more where the avast! definition updates are more than two days behind by not going on the internet to get the latest vps updates, I see exactly what you see. I feel this is normal behavior for an XP system that is still running the (in this case) the original creation volume from March 2008, so it has not had a fresh install in all that time. And it still runs well, even with the extra clutter one gets from running a system for so long a period without refreshing or starting over.
Then there is the issue of old hardware. The P4 processor is an example of that. When it was designed, it was pretty much state-of-the-art, but today’s software is much more of a challenge for it to run quickly and cleanly. It is much slower than the Win 7 system at the same tasks; about double the time to complete on average.
One way to avoid some issues with clean (fresh) installs with avast! is to always choose “Custom Install” and untick what you do not want at the beginning of the install process.
As for your HDD rating at ~3 MB/sec, that is definitely on the slow side. EIDE HDD’s can approach 40-50 MB/sec in burst speed, so you may have the old well-known XP bug: the PIO bug. Depending on how old your HDD is, age can also be a factor in performance.
Here is a link to a site that may help you with the DMA/PIO bug in XP. It just involves resetting a HDD setting back to where it should be, and once done, the system should run at normal speed once again: http://winhlp.com/node/10
In other words, if you find the HDD setting set to PIO, it must be set to DMA 2 or higher to get best performance. PIO is a fallback setting when XP has issues with the HDD failing certain consecutive tests; it is a safeguard setting allowing one to continue using the system as is vs. not being able to use it all.
Please report back as to what exact version you have now. You can do that by right-click the avast! tray icon in the lower right corner and selecting “About Avast!” Is it the free version?