Couple things:
1) What router do you have? Is it also from DLink? If not, be sure that anything related to speed enhancing is turned off in the router for the wireless.
Some manufacturers have their own special way of increasing performance with their own brand of cards. Sometimes they call it Speed Booster, XtremeG, or Nitro, however while these special products work with their own companies brand of cards they can interfere with others. So, the best thing to do is to make sure that these are turned off if you are using different wireless cards or routers with another different brand.
On the same note, you may want to try changing the broadcasting channels (try 3 or 11) as some channels might be better for certain cards, or forcing one mode such as b or g (for compatibility, or at the very least for testing).
2) Is your router fully IPv6 compliant? This is the new addressing protocol that Vista includes with any network device. If you take a look at your network cards properties you would see listings for the usual things, such as TCP/IP, QoS, and what not, but you would also see 2 very specific listings: IPv4, and IPv6. If you have a router that is not fully IPv6 compliant Vista can cause the router to... well.. misbehave. It can cause it to hard lock, drop connections, or reset connections. So, as a precaution you might want to disable it for the respective card.
For more information on what this is, and how to disable it, just take a look at the step by step instructions on
THIS page.
