Try disabling all aps in the Windows Startup Menu except for your anti-virus program, firewall, anti-malware aps that give realtime protection (Like Spybot if you have Teatimer enabled) and essential Windows aps.
I recommend Startup Inspector for managing the startup menu because it does a much better job of identifying the aps than the Windows Utility, msconfig does.
Startup Inspector – freeware (manages startup menu – helps you identify aps in the startup menu so you can disable unnecessary programs from running in the background when they are not needed)
Startup Monitor (a small freeware ap available on the same page to notify you and require your permission for any program to add itself to your startup menu)http://www.windowsstartup.com/download.php
What other devices do you use the serial port for? You might be able to use USB instead.
In any case, do as I suggested above and it may eliminate your problem.