Sean De Merchant , Oct 14, 2005; 03:10 a.m.
From a performance perspective it matters little. Unless you have a second physical drive using multiple partitions simply guarantees massive seek times when going from one partition to another.
With a laptop, using a single partition (C:) on the system and using an external Firewire (IEEE 1394) drive for scratch disk and perhaps a second external Firewire drive for OS SWAP would be the way to go.
As you situation stands, the choice matters very little.
Caveats given, IIRC, installing on the first partition created (likely C:) will yield slightly better performance. Albeit, where you install the application has less than 60 seconds performance gain on a system with a very slow drive. The location of Photoshop's Scratch Disk matters far more. The former affects application launch time, the latter affects how long operations that exceed your RAM take.
hope this helps,