Both have plenty of RAM. That will not cause a bottelneck with Windows XP.
The hard drive has very little to do with speed (most drives have similar access speeds/RPM).
Mostly likely, the problem lies in the processor. Nowadays, measuring speed by MHz or GHz is becoming almost irrelevant. GHz is just a measure of how many times the processor ticks per minute. In modern processors, they crank up the clock speed, but less is accomplished on each tick.
For example, take the comparison between the Intel Pentium III 500 MHz and an old Motorola (Macintosh) 500 MHz processor. The Mac will run circles around the Pentium because--despite having a slower clock speed--much much more is accomplished in each cycle.
Another example: A 1.0 GHz Pentium 4 processor and a Celeron 1.0 Ghz processor are not even comparable because the Pentium 4 is MUCH faster than the Celeron, regardless of clock speed. This is because of another factor: onboard cache. The Pentium has more built-in memory on the chip itself than the Celeron, making it much faster (and much more expensive).
Now, you haven't mentioned what type of processor each is. If one is a Celeron or Sempron, guaranteed it is the slower of the two, regardless of clock speed. If one is a dual core and the other is not, it might be faster. If one is Core2Duo and the other is CoreDuo, it might be faster. If one is Intel and the other is AMD, it might be faster.
In summary, processors are different. GHz is a completely inadequate measure for "speed" nowadays and has been for several years. The processor companies only crank up the clock speed as a marketing scam to make people THINK it's faster.