Odd Broadband Speed Problem

I have a wireless home network that permits sharing a fast broadband
connection between three computers, usually at separate times. Yesterday
after starting up my main laptop, I noticed the broadband speed was slow.
Testing the speed, it is one tenth of the speed I normally get. I checked
the two other computers assuming is was an ISP issue but the broadband speed
was normal. Diagnose and Repair revealed no network problem. On further
testing, if I restart in safe mode with networking, my broadband speed
returns to normal. When I restart Vista in normal mode again the speed is
throttled. There are no other apps that I am aware of that are using the
connection as my network meter gadget shows zero activity. I also disabled
my antivirus software to no effect. I'm not sure what else to try at this
stage. I have not installed any software in recent times so I'm not sure a
system restore would be of any help. Any ideas?? Thanks - pbl.