2 scenarios
1. Office - No problems for laptop connecting with Linksys Wireless AP.

2. Home - Laptop connects to Belkin router when hooked up on LAN. Will not connect on WLAN. The laptop can't even pick up the signal even when it is side by side with the router. 2 other laptops connect OK even though they are in different parts of the house. Windows reports "No wireless networks found in range". Occassionally i will see the laptop WLAN signal go green and it will attempt to get an IP address from the router but when that fails, it reverts back to "No wireless networks...".  Very infrequently, it connects OK and will be assigned an IP address but drops out shortly after.

Laptop is running Windows XP.
Security - WPA-PSK at both sites
Firewall - MAC filter at both sites
I can't see that this makes a difference as the laptop can't even get the signal, much less worry about how to talk to it.

If it makes any difference, the laptop is running Check Point VPN-1 SecureClient and the Security Configuration is Verified.