Hi colinsw,
Excuse my English communication... normaly I speak and write French.
If you have a data (document, picture, download, muscic files, etc) ... save it on other hard drive or USB Key... or on the same hard drive, but in other partition than OS System.
Yes if you have a time and patience, this is a good idea cleaning Hard drive (but not partition with DATA....)
The clean is simple : When you install Windows Seven, by exemple, you have a Button ''Options'' in blue characters. Click on. And normaly, you view all partitions and all hard drive, IDE, SSD, SATA....
So Just make attention do not Erase a partition or Hard Drive with a DATA.
So When you choose a partition or Hard Disk for install OS system, you have a possibility to Format a partition before. Or Resize the partition or Delete the partition...
One idea, If suppose you have installed two or more Hard Drive on system. If you have Saved Data to HD No 2, by exemple.
When you are ready to install Windows on Hard Disk 1... Before you have a possibility to unPlug, remove a electrical cable connected on pins Hard Drive 2. So This hard drive is protect againt ''Operation install Windows''.
After finish install Windows on Hard Drive 1, RePlug a electrical cable on Hard Drive 2.
Important : When you remove or plug a electrical cable on system.... Turn off the computer, Unplug a main cable power from wall. You have a possibility to switch off the PSU with button in back.
If is not clear ,Take a time, return here with comments.
Gloup_Gloup