to clear ip stack:
http://support.microsoft.com/kb/299357
btw even after a period of two years without regular maintenance on any pc...could cause system degradation.
when talking about lag there are multiple issues.
sometimes just restarting the computer will help others just do a full system defrag and disk cleanup. if you have any anti virus software run a scan and see if anything pops up.
however, in the best circumstances even if it has always worked...doesn't mean it will always work. Just have to figure out what is causing it and clear it out.