I recently installed a fresh copy of Windows XP on a new hard drive, but I had this problem before that, although not quite as severe.
My Belkin N Wireless router is about two floors down, although signal doesn't seem to be a problem. I'm using an EDIMAX wireless-n PCI card on my Dell machine to connect to it. Signal varies between good and very good so no issues there.
I use my Macbook pro and my iPhone in the same room, and the internet is perfect and fast. My desktop on the other hand doesn't. It might take a couple of refreshes for Google to load up for instance. It's the same with most sites, they take about 3-4 clicks of the refresh button, and even then sometimes the page may not load. Installing things like Firefox extensions will hang. Repairing the connection will let it work for about 15 seconds at most.
As I said, the signal is strong, and I can't see that there's anything wrong with the router as my laptop and phone both work on wifi perfectly.
Has anyone run into this problem before? Any suggestions would be really appreciated.
EDIT: by the way, clicking on any web link or bookmark will cause the ZoneAlarm activity meter in the taskbar to start jumping about, for about a second, then revert to the 'Z' logo, ie. no internet activity. It's as if the computer tries loading a website for a second, then doesn't receive any more data after that.