I only have scan for modified and created files and it scans like over 900 files. I don't have it setup to scan on start or anything else. I just don't see why xp would create or modify over 900 files on startup.
What other security software do you have running on boot ?
I have seen something like this before and it was my firewall's anti-spyware opening all and sundry, adding hundreds of additional files to the scanned total. I promptly disabled my firewalls resident anti-spyware and that reduced the scanned count to 150-175.
This was a long time ago with an earlier version of avast, currently with this version and I don't have my scanning restricted as you have and my scan count tops out at just under 200 and (still bots in under 1 minute) I have SuperAntiSpyware as a Resident anti-spyware running on boot also.
Personally I wouldn't be looking at stopping avast scanning (hiding the symptom) on boot but trying to find out what it is that causes avast to scan the files in the first place (find/treat the cause).
Perhaps by logging what is scanned, though I'm not sure how this would be don during boot.