Sorry to say but USB wifi adapters have pretty much always been flaky at best. Of course in the case of communal wifi, you can't rule out things like a poorly configured access point, or some idiot in another unit running half a dozen file sharing programs causing problems for everyone.
To start with, you should probably get a wifi card that actually goes inside the computer. PCI for a desktop, PC Card for laptop. Make sure your laptop has a PC Card slot first though.
Also, might be best to avoid D-Link and Netgear. They have a kind of reputation for being cheap in the not so good sense of the word. Linksys isn't necessarily always any better, and if anything, the Cisco takeover has only made things worse. But like most things in life, you get what you pay for. Doesn't leave you with a lot of options, I know, but such is the sorry state of consumer electronics these days. Everything is mass produced in Chinese sweatshops, where people work 12-15 hour days with little to no rest. You can imagine how quickly fatigue likely sets in and starts affecting quality.
I never thought this could be so difficult. I have a gateway xp with service pack 3. I bought a D-Link DWA-125 wireless N300 USB adapter, which the specs says uses xp with service pack 3. Great, I live in an apartment and wifi is supplied.
I installed my new adapter and it works great. I have a strong signal and it connects to the apartments wifi just fine. The problem is, it keeps disconnecting in about 4 seconds.