Every time this problem has cropped up, it's tended to be related to the encryption. Take it out and see if the problem goes away in the clear. We can work on solving it from there.
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I found the following which might help:
"The quick installation guide implies Windows XP will detect the card and set it up automatically. It did no such thing for me; it wanted me to provide the driver manually. I highly recommend getting the latest driver from the Linksys website rather than using the one on the CD.
I don't recommend using the Windows network wizard, or whatever the thing is called. It added a Network Bridge device with an odd IP address. I could not communicate with my router until I deleted this device. This was not intuitive at all, but again this is a Windows problem.
On to the next problem: when I used WEP encryption (either 64 or 128-bit), the wireless connection would drop out at precisely three minutes. Everything worked fine with encryption turned off.
I then decided to try WPA encryption, which is supposedly more secure than WEP anyway. There is a Windows XP update to add this capability to the operating system, which you'll need to install. Unfortunately I found that this didn't immediately solve my problem: WPA was not available as a selection in the wirless network setup. I then updated the driver for the WMP54G and I was able to use WPA with no problem! Again, a word of advice: do NOT use the driver on the included CD! Get the latest driver off the Linksys website. I would imagine this was the source of my WEP problems as well, but I never went back to try it. I'm happy with WPA.
Also make sure your router's firmware is up to date. Some manufacturers have only recently added WPA capability."