Hello Sinh, Welcome to TSF!
Before we get too carried away, there are a number of possibilities that might cause this issue.
If there is only 512Mb RAM installed, chances are that about 64Mb of it is being used for the on-board graphics (if you don't have a separate graphics card), and this will mean that in reality there is only 448Mb of RAM available to operate the system.
If there is a high end graphics card installed, this will further add to the 'slowness'.
Having said that, please have a look at these articles...
1: Run the Windows® XP Disc Cleanup Tool. (
Click the coloured link.)
and;
2: Is your PC running slow? (
Click the coloured link.)
When finished, please post back with the results.
Kind Regards,