World of Warcraft Lag?

I play world of warcraft burning crusade, when I first start up my pc and the game it works fine and runs smooth, however after playing for about 30 Mins everything begins to lag. I restart my pc and the game and it runs smooth for another 30 mins or so then more lag. The whirring in my tower gets louder right before it happens, I dont know if this is the reason. I have all my setting's set on low in the game. my specs are

OS: Vista Basic
Processor: Intel celeron D CPU 3.20 GHZ
Ram: 1518
System type: 32 bit
Graphics Card: Radeon x1300

Property Value
Product Name W3502
Serial Number CRC67 200 05912
Chassis Desktop
PC System Type Desktop
Machine Type AT/AT COMPATIBLE
UUID 0AF38E24-41D2-DA11-A3C0-000EA68F7...

Windows System Assessment
CPU Score 2.00 (Calculations per second)
Memory Score 2.20 (Memory operations per second)
Graphics Score 5.90 (Desktop performance for Windows Aero)
D3D Score 3.60 (3D business and gaming graphics performance)
Disk Score 5.50 (Disk data transfer rate)
Windows Experience Index 2.00 (Base score)

Disk Space Disk C: 166 GB Available, 298 GB Total, 166 GB Free

Physical Memory 1518 MB Total, 868 MB Free
Memory Load 42%

Virtual Memory 3275 MB Total, 2406 MB Free

PageFile Name \??\C:\pagefile.sys
PageFile Size 1818 MB
In use 84 MB
Max used 88 MB

Minimum File Cache Size 1 MBytes
Maximum File Cache Size 2052 MBytes

Registry Size 15 MB (current), 682 MB (maximum)

Profile GUID {d8932e40-6a6f-11db-b6ab-806e6f6e69...

The system clock interval 15 ms

Warning! Accuracy of DMI data cannot be guaranteed

Its probably due to the constant fragmentation, uploading, and downloading of data. WoW transfers alot of data in the background while your playing. I know some people who have to defrag weekly to keep their game running smooth.

Have you tried defreagging your machine?

Yeah definetly Defrag your computer...Cacheing-downloading and uploading...in the background...like Shaun W said ... If you've ever played Oblivion you know wat Cacheing is...good luck...Wats your realm?