If it happens everytime during bootup, then it could be a service, program or driver issue.
Open your Run command and enter "msconfig" without the quotes and hit enter.
On the services tab, click on "Hide All Microsoft Services" and reboot. See if it stays connected.
Then run the Startup tab. Disable all. If it stays connected, then reboot and disable five at a time until it disconnects, then troubleshoot those five.
Under General tab, click on Diagnostic Startup and check again. If this changes things, it may be difficult to find out which driver.
What antivirus are you running on the clients. Are they different on XP than the WIN7s? Disable the Antivirus and/or firewalls on the WIN7s and reboot.
If you are running McAfee or Norton Suites turn them off. And, turn them off by uninstalling them. Especially if you have McAfee or Norton on the WIN7s and not on XP. And, ESPECIALLY, if you have McAfee.
