Hi and be welcome!
This problem you are experiencing could indeed be related to the harddrive you are installing Windows to, but it doesn't have to be like that. There are quite some reasons why a system can be slow. It is quite common that Windows gets slow after being installed for some time, esp when there's loads of software on it. If you are reinstalling from a CDROM that is hard to read (scratches, dust, grease etc), it may take a long time to read all the data. This is just an example situation, but it could well be the case.
So let me ask you a few things:
1. Is your primary master really a 4200rpm harddisk (could be a typo, as 7200rpm is common for regular disks, 4200rpm is something we are used to from 2,5" disks)?
2. What exactly is running at a to low speed.. is it the harddisk? Or does this involve tasks that require heavy computation?