First is helps to understand the signal radiation pattern for a typical dipole antenna that us used by most 802.11 routers and adapter cards. The radiation pattern looks approximately like placing the antenna through the hole in a big glazed doughnut, where the doughnut represents the signal. Lots ot signal strength all around on the sides, but little top or bottom. I assume you already tried moving the router and adapter antennas around though.
There are a couple of things you can do depending on whether you have a problem with one PC, or all. It you are having trouble with one PC you can easily replace the adapter card antenna if it is removable and has reverse SMA connectors. Hawking Technology, Linksys, D-Link, and Netgear all make these antennas (I helped set up a friends computer with a Hawking Technology 15dBi directional antenna). If the problem is with all PCs, then try replacing the antenna(s) on your router.