I added three old hards drives internally to my PC - there are now five hard drives in total inside the tower.
Does running this many hard drives slow down PC performance? Is there any way I can turn on/off hard drives from inside Windows XP?
It should not affect your system's performance (outside of having to load a few additional drivers), unless you are accessing all of the drives simulatenously. In that case, your drive interface bus (IDE/SATA) may saturate, causing the transfer rates to the drives to slow down.
If you wish to disable some of these drives, you have two options. You can disable the actual hardware by going to Device Manager and disabling the hard drive device itself. Alternatively, you could go to the Logical Disk Manager, and either revoke (i.e. unmount) the drive letter from those particular drives, effectively limiting your access to them. Then, if you need access to the drive in the future, you could just set it as a different drive letter.
It shouldn't slow the performance of the system.
There aren't that many operations that will scan all of the drives in one go.
A system anti-virus scan might, but you could tell that not to look at one or more of the drives. On-demand anti-virus scanning won't be affected as that only looks at the files you are actually reading/writing so will be consistent where ever the file is located.