Slow Internet on 2 of 3 computers.. why?

[PatInTheHat]

Ok so I have 3 computers on a 2-3 year old DLINK router. During the day I get great speed test times and everything seems to work fine. Without fail almost every evening 2 of my 3 computers start absolutely crawling.

Using the same speed test site I am often getting 100-300 kbps on my two XP machines and 5000+ kbps (which is closer to where I should be) on my Vista machine.

I am absolutely baffled as to why this might be. Anyone have suggestions/ideas?