Download & install latest firmware version on router.
Download & install latest driver for wireless network adapter.
Run Windows Update and install SP2 & all critical updates.

Install and update personal firewall, antivirus, and antispyware.
Scan with updated antivirus and at least 2 antivirus programs.

Router make & model?
Wireless network adapter make & model?
Operating system on wireless computer?

SIGNAL STRENGTH:
How far is router from wireless computer?
How many walls, material, and how thick?
Any 2.4GHz phones or microwaves near router or wireless computer?
Do you have the same problem 10 ft away and in same room as router?