I found this in a few different places around the 'net - hopefully it's helpful:

Very Slow Boot When Networking

On some XP Pro installations, when connected to a network (peer-peer in this case), the computer boot time is over 1:40. The system seems to freeze after logging in and the desktop may not appear or will freeze for a minute. As timed with the utility, Bootvis.exe, the problem was with the driver mrxsmb.dll, adding over 67 seconds to the boot time. Turning off and restoring file and printer sharing eliminated 65 seconds from the boot time.

• Alt-click (or right-click) on Network Places > Properties.
• Alt-click on Ethernet Adapter connection > Properties.
• Un-check "File and Printer Sharing for Microsoft Networks" > OK.
• Reboot.
• If you need file or printer sharing, repeat the above, re-check the box and re-boot again.