Firewall won't launch, avast blocks 100% systems use

I am trying to install Windows XP OEM SP1 (or SP2) + Avast! Home version 4.8.1229 + Kerio 2.1.5 (latest free version available)

I know very little about Windows XP!
1) I installed Windows XP first AND then SP2
2) then all the necessary software
3) then install the firewall (Kerio) and branch machinery on the ADSL network

So far so good, the machine works perfectly OK. I choose filters in Kerio to allow Firefox etc. Finally installing avast, which initially settled properly. Authorized avast, in Kerio. In the first reboot everything works well. Then I shuts down the machine and restarted, and ....

1) the firewall will not launch any more (so I pull all Trojan worms etc etc)
2) avast! blocking the machine, using 100% of resources

What's going on in your opinion?

How I performed it:

- Insert the disc and install Windows from Windows CD by creating partitions (at least C: first)
- Windows Firewall enabled
- Connection to a secure network (behind a firewall)
- Update Windows (several times!)
- Install antivirus + reboot with test
- Install firewall
- Formatting partition D:

I have no problem with this method.

Thank you for your answer! But my problem is resolved: it seems that I used a utility to Edit the registry which was contaminated by a virus.

Are you really using OEM version?

Certainly I do not use any utility that modifies the registry. An update would not have happened that! Moving My Documents to D: \ (user) \ makes sense! But there are other elements within Documents and Settings \ (user) who do not have to be moved! What to change the name or place of "Program Files"?