Why Parallels so slow on one computer and not another?

When I installed Parallels with Vista on my new Macbook, it ran (runs) perfectly with no noticeable slowing down of my computer.

When I installed Parallels with XT on another, slightly older Macbook, it makes the computer run at a snail's pace no matter if I am working in a Mac program or a Windows program. Why does Parallels run so slow on this computer and is there any way to speed it up? Its specs are:

Model Name: MacBook
Model Identifier: MacBook3,1
Processor Name: Intel Core 2 Duo
Processor Speed: 2.2 GHz
Number Of Processors: 1
Total Number Of Cores: 2
L2 Cache: 4 MB
Memory: 1 GB
Bus Speed: 800 MHz
Boot ROM Version: MB31.008E.B02
SMC Version: 1.24f2
Serial Number: W8743DRHZ63
Sudden Motion Sensor:
State: Enabled

 

Most probably the memory. If your XP is set to use 512 MB or more, too few is left for good performances on Mac OS X side.

The newer configuration has most probably 2 GB or memory instead of 1. Solution: just add memory to upgrade it to 2 GB.

 

thank you!