I just told you one of those ideas... avast! mail handling under Windows XP works different than under Windows 98/98SE and Windows ME. That's why mail scanning consumes little more time... unless you are getting internet connection timeout, everything is completely normal.
I have 5 different e-mail accounts. I feel free to say, no one receives more e-mails than me. My customers sends me more than 50 e-mails every single day, and that's just one of those 5 e-mail accounts covered with avast! mail protection. I never experienced any delay in receiving and/or sending e-mails, whether I use OE or MS Outlook 2003.
If you still think it's not how suppose to be, try repairing avast! installation, or even better, uninstall it completely from your system and install it one more time from the scratch.
Another thing,
you still didn't say anything about your e-mail accounts, which e-mail providers you are using services of. Every single information is important. Maybe some of those e-mail providers already have antivirus checking on their servers. That can also drastically decrease download time, especially if those e-mails are being checked twice...
Also, what's the name of e-mail notification program that you are using ? How did you set all those POP3 server names ? It could be that everything was properly set when you were using it on WinME, but now, you may need to change some settings. Don't forget that under Windows XP you don't need to have 127.0.0.1 instead of original server names... but of course, I already mentioned that in my previous post...