Why Registered Window has Single partition ?

What are you asking? I don't quite understand your problem or concern.

If you are asking why systems are sold with a single usable partition is because it's easier to manage for users. A big portion of the population that uses windows only know how to do their word/excel documents. They don't mess with default save location or anything like that. Go out an explain partitions to someone who 1) doesn't really care 2) is not very computer literate 3) will forget. They are comfortable with a single partition to save and access their documents.

Well the majority of PC users don't bother maintaining their systems and either a) download 3rd party software which sometimes works sometimes it complicates b) take it to a shop c) know someone who can fix it.

I don't think having a second partition would help them very much. The performance gain vs complexity is just not worth it. Just imagine the number of increase calls for a PC manufacturer, there would be an increase number and that's just more money they need to spend. They spend more money = more expenses = they pass the cost down to the consumer = more expensive computers for the consumer.

The performance gain is not significant

mmnouman said:

I inquired because I found that single disk partition slows down the performance.

Could you explain how a single partition slows down a computer?

You can "short stroke" the OS partition.

I read an article where the author compared a small WD Black (250GB) HDD to a large WD Green HDD (1 TB).
His tests showed, the WD Green was almost as fast as the WD Black, if he created a small partition for the OS (and left the rest as a large data partition).
The large WD Green was also cheaper than the small WD Black.

I don't have the equipment (or inclination) to confirm his findings.

You also seem to get more fragmentation if everything is on the same drive.
All the various Windows (and program) log files and updates get scattered everywhere.