Vista has an auto tuning feature which is hit and miss, depending on what network appliances/cards/devices you've got hooked up. Try this:
1. Navigate to Start>Programs>Accessories>MS Command
netsh interface tcp show global
3. Look the line for receive window auto tuning. If it says highlyrestricted type this command (all on one line):
netsh interface tcp set global autotuning=restricted
4. Try browsing, if it isn't any better, enter the same command like, just change restricted to disabled. Try browsing again, if there is still no improvement, change disabled to normal.
For those of you interested in the techie details, check out this blog: