I am afraid that you have provided us with very little information about your problem :) .
Let us know what OS are you running and what router and NIC (the exact models) are you using.
If you are using any Internet Security software, disable it (or at least the Firewall).
For Windows XP : - make sure that you have SP3 installed (there are some issues with XP and the 802.1X wireless protocols).
- Click Start, point to Connect To, click Show all connections, and then double-click your wireless network adapter. On the General tab, click Properties. Click the Wireless Networks tab. Under Preferred Networks, click your home network, and then click Properties. Click Data encryption (WEP enabled). Click the Association tab, and then click to clear the Network Authentication (Shared mode) check box and the The key is provided for me automatically check box, if they are selected. Click the Authentication tab, and then click to clear the Enable IEEE 802.1x authentication for this network check box, if it is selected. Click OK two times to accept the changes.
For Vista/ 7 make sure UAC, Defender and Windows Firewall are disabled.
If this PC is located in other room than any of the other three, check for any possible sources of interference (2.4Ghz cordless phone, microwave oven, another wireless network with a stronger signal).