Here are a few suggestions to improve signal strength for your wireless router:

1. Try to move the router to a different room.
2. Check to see if you have a cordless phone in the general area of the router since both a router and a cordless phone may work on the same frequency.
3. There are certain antennas you can purchase to improve the signal of the wireless router.
4. Make sure the antenna of the router is fully extended.
5. Try reducing the distance between the router and the wireless card on the PC.

About.com has a good article on tips for improving a wireless home network -

Top 7 Tips for Improving a Wireless Home Network