Data yes, Programs NO.
The Programs are dependent on the Registery.
A repair to the OS will indeed be easier.
A re-install of the OS and even Programs on a 3rd partiton will not effect
you data partition.
Sizing the OS partition is very very easy and logical.
From many many example the OS with all programs, page file of 1.5 GB,
restore points of 1 GB and 2 GB of free space hardly ever exceeds 12 GB.
NTFS really like 16 GB and if you want to install every copy of Adobe
software ever made and 10 large games, and 50 regular programs with OFFICE
2003 PRO and 20 utilies, the space uesed will hard be 32 GB. So what's to
think about.
Pick a C: system /Boot partition size 16, 20, 24, 32, 37.5)
Then partiton the rest of the disk and / or a second disk any way you like
for you fav space management style.
SJ