Ok here's a list to follow since we're going nowhere on this one at the moment:-
1. Has this setup ever worked successfully with good signals in its current location? If yes, what changes happened around the time it became unreliable?
2. If you have a cordless phone, try temporarily switching it off and unplugging the base unit.
3. Do you have a mobile phone mast on or close to your building, an aircraft or radar station next door, radio ham or cb'er in the building, or anything else likely to be pumping out RF?
4. What channel is currently being used? Assuming there are no other neighbourhood WiFi LANs showing up, move channels as far away from the one you're currently using.
5. Try temporarily moving one of the PCs much closer to the router, and see if this makes any difference to the received signal level. For just 15ft away line-of-sight, I'd be expecting a full 5 bars on the signal strength indicator.
6. If you have any security set on the units, (WEP, etc.), temporarily disable it so it's not confusing the issue.
7. If you're running any firewall software, (Norton, Zone Alarm, etc.), temporarily knock this off, ditto No.6 above.
8. Try uninstalling the software and remove the PCI WiFi card. Reinstall card in a different mobo slot, reload software.
And finally, before flasing the router, check with your supplier to see if these devices are still within their warranty period, as it's possible one or other may be just plain faulty.
ROGER - G0AOZ.