It’s doubtful the reason is because you are using WEP. WEP might add some overhead, but it would generally stay constant and not slow down over time.
It’s more likely you have a bottleneck elsewhere or possibly some spyware on your system.
First off, it is easy enough to get a copy of Spyware Search and Destroy or lavasoft’s AdAware Personal and scan your system for spyware. I’d start there, assuming your C: drive isn’t full.
I would also try these things:
Defrag your hard drives.
Scan for Viruses
Check CPU Usage
Check memory usage (Task manager can give you a 40,000ft view of memory and CPU)
Check your connection speed. (Speed Test is a good site for this.)