Unable to use WEP

I am using Microsoft MN-500 wireless router with Intel PRO-2100 wireless
card in my Dell laptop.

My wireless internet is working just fine with WEP disabled. As soon as I
enable WEP (64-bit, or 128-bit), my wireless connection starts dropping
every 3 minutes and 10 seconds. I also have a desktop with Microsoft USB
wireless desktop adapter and it does the same thing when I enable WEP.

Microsoft help line is not available saying "they are experiencing very high
call volume"

Any suggestions on how to enable WEP and solve the problem of dropping
connection ?

Thank you for reading the message and any suggestions.

Mayur