CISCO WRT310N 300MBPS won’t work if WEP Security is used

Hello Everyone,
I have a WRT310N with a wireless Lenovo Laptop (windows 7) with wireless a/b/g/n and an older Desktop (windows XP) with a wireless B adapter (Linksys WUSB11). My Lenovo laptop can only get 54mbps of wireless speed while my windows XP gets 11mbps(i think this is limited using my wireless B adapter) using WEP 64bit password. According to my Cisco-Linksys router box, the speed can go up to 300mbps using WPA2. When i used WPA2, my windows XP desktop won't connect wirelessly.
Your Response is highly appreciated.
Francis
