1) Check your signal strength. If low, a vertical [i]booster[/i] antenna on your router can extend horizontal Wi-Fi range by ~50%
2) Check for 2.4GHz interference, such as a cordless telephone base station
3) Check for interference from competing signal. If another router is broadcasting with comparable signal strength, there may be interference if the channels are close together. Many wireless experts recommend at least 5 channels between competing signals due to bandwidth issues.
4) What type of encryption are you using? There are issues with WEP that are sometimes resolved with WPA. Does the problem vanish when you disable encryption?
5) If the problem seems related to encryption, try this workaround - disable encryption, restrict network access to MAC addresses of authorized computers. Monitor network activity. If you detect an intruder, kick him off the network by entering his MAC address in the blocked MAC address list (I use the masculine personal pronoun, because we all know that ladies would [b]never[/b] do such stuff).
6) Is your wireless adapter built in? Is there an alternative to the native driver? Sometimes the 'default' Windows Wi-Fi driver works better than the proprietary driver.
7) If your wireless adapter is built in, and none of these suggestions corrects the problem, consider a Linksys 'Speedbooster' USB wireless adapter. That was my workaround, and I'm much happpier with the Linksys system.
Hope this helps.
Rick/Portland, OR