Does the startup lengthening occur before or after the desktop first
appears?
If after, check out what you have running at startup. You can launch
MSCONFIG from a Run box and check the Startup tab, or look at this site: http://aumha.org/win5/a/loads.htm
Another thing to check is network connections. A failed network connection
trying to be made will keep retrying for about 3 minutes before it times
out.
You can also use MSCONFIG to have Windows create a boot log at startup, then
examine this for delays.