David R, what do you use to find adware on your computer? I've been using AdAware for a long time, but if there's something better that's still free, I'll use it. I also use Spybot to find spyware on my computer. I usually pick a day once a week & run Avast, AdAware & Spybot to keep my system clean.
You can see my signature in my posts, I use MBAM as a secondary anti-malware scanner on on occasions run an SAS on-demand scan. I have had both for some considerable time and neither have found anything, but I don't really expect them to.
You will also see in my signature that I also use XP Pro SP3 and no issues, so it isn't an issue across all XP systems.
You didn't answer the question in my last post as to why you can't install avast again (e.g. what errors do you get) ?