Slow speed of 3G Dongle

Hello Everybody

Recently I have purchased 3 Dongle which says Download speed upto 7.2 Mbps. When I use it in my PC I am not getting speed over 250 Kbps. When I checked it on another Laptop I got speed upto 3.2 Mbps. My system configuration is Pentium Dual Core Processor with 1.6 GHz speed, RAM of 1 GB. I am using Windows XP (Service Pack 3). The Laptop on which I checked was running on Windows 7 with 4 GB RAM. Processor was Intel Core i3 and 2.4 GHz speed. What should I do to get better internet speed?

Ravindranath