Windows 7 installs with varying partitions

[John-stpaul]

I re-commission a number of business class mini-tower computers (HP & Dell mostly) re-installing Win 7 or replacing XP with Win 7 pro. Most of the time the Win 7 install protocol creates a small partition 0 (for certain system maintenance functions, I'm told), about 50 MB, along with a partition 1 of specified size when the system installs. Sometimes Win 7 does not create the small partition carved out of the partition 1 "C:" partition.
I'm trying to figure out why it makes the small partition much of the time, but not always. Hidden switch somewhere? Does it have anything to do the kind of hard drive, or the way the drive is formatted or re-formatted before or at the start of the install l process?
I'd like to be able to produce consistent installation results.
Advice appreciated.

 

[USAFRet]

In some of these devices, are there multiple drives installed?
If so, that small System Reserved thing ends up on the secondary drive.