Hello
I think I had the same problem.
I needed to 'repair' my internet connection every 15-30 minutes with my DSL connection (and disconnecting the modem when I use mobile broadband connection).
First I thought it was because of firewall problem. I used Comodo Internet Security, firewall and defence+ only.
Unistalled it and tried other firewall (Online Armor, PC Tools, and Private Firewall).
Still got the same problem. I did repair the XP W32, DNS cache, TC/IP repair etc, no success. I almost blamed my modem
Finally I turned Avast web shield of then everything works fine until now. My connection never goes down.
I currently use Avast 5, Online Armor Free, and Win Patrol. MBAM and SAS for on demand scan only. XP Pro SP3.
Turning off the web shield might be not the best solution for me. But I do like Avast a lot so I keep using it.
I've been using Avast for years.