Unfortunately it is usually the disk that is the slowest link in the chain, as the OS demands more from it, the more work it has to do, the more the heads move and the more the data queues up. Moving it to another system or even disk wouldn't really help matters, unless the hardware is already really out of date. If it would fit onto an SSD on the other had, things would be very different.
The other issue is that XP's licensing wouldn't allow you to change too much hardware, although you may be able to get around that with a call to Microsoft, depending on whether the version of XP installed on the system was a retail version with a disk or on an OEM License only to be installed on the one machine.
What hardware is already in the system and what do you intend to upgrade it to?
Also what does the system get used for?