I would start by looking at the signal strength of your wireless connection - as you mentioned you can sometimes get a connection - albeit slow and unreliable; it may be that your wireless box isn't outputting a strong enough signal to reach the PCs. Ceilings and proper brick walls all contribute to slowing the signal. If you have Windows XP Service Pack 2 - have a look in the Wireless Connection box. If you don't have SP2, try installing all the software that came with the Belkin dongles. In either case you are looking for a signal strenth bar to show you how well the signal is getting to you.
If you can get any connection at all - I wouuldn't start toying with IP addresses.
If the signal is poor - you may have to invest in a more powerful wireless box - alternatively try re-positioning the box closer to the PC's.
Best of luck