A bit more detail will help. Can you please answer the following questions:

 

System information

1)      What O/S - (XP, Vista, 2K)

2)      Was the system upgraded? If so, from what to what?

3)      What kind of a card is in the system (Intel, Dell Wireless, etc.)

4)      What driver version is currently installed?

5)      If in XP what wireless utility version is installed?

Router/AP info

1)      What is the make of the router (Linksys, Netgear, Belkin)?

2)      What model number?

3)      Has the Firmware been updated? If so what is the firmware version?

Additional Info

1)      Was it working before? If so, when did it stop working?

2)      If you upgraded your system did it work in XP?

3)      Have you tried in another location?

4)      Do you get any error messages?

5)      Has any hardware been replaced?

6)      Can it see any networks?

7)      Security, if so what kind, is it WEP, WPA, WPA2, MAC Filtering, etc.?

8)      Is the SSID broadcast or hidden?

9)      If running Vista have you set up the profile manually and checked the following boxes:

a.       Start this connection Manually

b.      Connect even if the network is not broadcasting

10)  What firewall are you running?