Client connections do not authenticate successfully with a Cisco switch when it takes more than five seconds to generate an NAP SoH message on a Windows Vista-based computer or on a Windows XP Service

When it takes more than five seconds to generate a Network Access Protection (NAP) statement of health (SoH) message on a Windows Vista-based computer or on a Windows XP Service Pack 3-based computer, client connections do not authenticate successfully with a Cisco switch.

The default supplicant time-out is 5 seconds on Cisco switches. If a computer takes longer than 5 seconds, the computer will not authenticate successfully. Thankfully, just upping the time-out value on the Cisco switch fixes this problem.

Read more about it here - KB 947017.