Not really,

It may be easier for some to deal with two partition specially when doing
maintenance like defrag and back up.
You could have only the operating system and those files which is
automatically installed on the primary partition on C and put all your data
files on D.

However, I feel the primary reason would be how you prefer to organize your
information and how you prefer to interface with your computer.

One of the key reasons is for imaging - Programs like Drive Image or Ghost
cannot image the Windows installation onto the same partition.

If the disk is a large one (say 80GB or more) I'd be inclined to have two
partitions anyway. That's my personal view, other may not agree of course, it
depends on how you use your computer.  

2 reasons. One reason being that if you only have one HD and want to be
able to rebuild the system when things go pear shaped you've got the
much larger second partition to keep all your data on whilst doing the
rebuild on the system partition. Second reason being if you use Ghost or
something like that to keep a good "starting point" image then you need
to have it on a separate partition/drive.

Personally I reckon drives are so cheap I just have one 200 gig as the
system, another 200 gig as the data drive, hate messing around with
partitions.

The One partiton it's there so you can restore your PC ( to factory
condition) if it crashes.

Remove the partition and computer crashes , you're hosed!!

Ask someone with an HP Computer (maybe its Dell, whatever)!!

Definitely could be asking for serious trouble!!
HIGHLY recommend that you check with computer MFG tech support!!