I had this same issue this evening.  I just updated an Windows XP Professional Machine with Service Pack 2.  Now, the wireless network won't connect and was stuck on “Acquiring Network Address”.  Even though I could see that my router had assigned it an IP address is still wasn't connecting.

Googling provided this post in a Linksys forum.  Which points to this update for Service Pack 2.

After updating that machine and getting it to work I was reminded by my darling wife that her laptop's wireless was flaky.  Actually, it was flaky to the point where I had turned wireless off and connected her to the router with a cable.  So I applied this same update a few days ago and she's not had any issues.

Giddy Up!