Slow Internet

I'm using Firefox 30.0. The longer I have the browser open, the slower it gets. I've had the same problem with the last three versions of FF. When the browser is closed, our PC is still slower than it should be.

I did a speed test:
http://www.speedtest.net/my-result/3607927064

Here is my sys info:
Tech Support Guy System Info Utility version 1.0.0.2
OS Version: Microsoft Windows XP Professional, Service Pack 3, 32 bit
Processor: Intel Pentium III Xeon processor, x86 Family 6 Model 23 Stepping 10
Processor Count: 4
RAM: 3037 Mb
Graphics Card: Intel(R) G41 Express Chipset, 1 Mb
Hard Drives: C: Total - 473300 MB, Free - 302591 MB;
Motherboard: LENOVO, To be filled by O.E.M.
Antivirus: McAfee Anti-Virus and Anti-Spyware, Updated: Yes, On-Demand Scanner: Enabled