I used to have wireless lag on XP, to get around this issue I disabled Windows XP's Wireless Zero configuration and used the Belkin software to connect to the router - this stopped the wireless lag.
Now, I have Windows Vista and after a bit of research I've found out that you can't disable this same service as it completely cuts out the wireless connection. Also, as far as I know there is no Belkin software compatible with vista yet. However, if I download when playing a game I know longer get lag spikes every 60 seconds - thought this might have some relevance.
By the way, I'm using an external Belkin G+ MIMO USB adapter to connect to the router
These big predictable spikes are usually Windows searching for "better" wireless networks every 60 seconds. I had the same problem with a Linksys USB adapter using Vista's drivers before upgrading to Windows 7. There's a program out there called "Vista Anti-Lag" (I don't remember the site off hand) which will manage your wireless connection and worked great in my case, though some people have said it makes no difference. I'd try that out.
Also, if you start getting overall higher latency even without the big spikes there's some additional things you can tweak to make the connection in Vista or Windows 7 smoother - I've detailed the steps on my blog: