Partitioning can be quite beneficial, even if you're a complete idiot :)
The first section of an HD is the outer rings, which have more data per rotation. That means they are 35%-100% (depending on the HD model in question) faster than data at the end of a drive. If you put your must-be-fast data in a partition at the beginning of a drive, and your mp3s/sundry items at the end of a drive, you're making good use of your partitioning.
Also, partitioning aids file access by having separate file allocation
tables, reducing the amount of files per file allocation table and speeding up
file access overall (if done well).
Having two drives benefits because you cannot read and write from a hard drive at the same time. So rendering audio from one drive to another saves time. This is somewhat complicated by (thirdly) sample-reading and (fourthly) page-file reading.
A good setup:
Partition 1: System OS partition - windows, apps, nothing else
Partition 2: Plugins
Partition 3: Audio projects
Partition 4: sundry items (mp3s, media, documents etc)
Partition 1: 3-4GB virtual memory pagefile/temporary files partition.
Partition 2: Samples/Rendering/Output area (ideally on a third drive)
Partition 3: Archival/backup area
Partition 4: Program installers/drivers
Drive 3 (optional - eliminate partition 2 on drive 2):
Partition 1: Rendering/output area
Partition 2: Samples
Partition 3: System partition backup
The fact is, the system OS only gets used during boot and while loading apps. Yes there is some occasional OS logging and 'housekeeping' happening, but if your 'temporary files' locations (under xp this is configured under control panel/system/advanced/environment variables/temp & tmp) and internet browser cache locations are set to another drive (as in the above scenario) typically this is negligible and does not affect performance.
Once the app is loaded, -then- plugins get loaded.
Once plugins are loaded -then- audio file access becomes important.
So it's not going to make a difference to have system, audio and plugins on separate drives from each other (if you have plugins which are essentially romplers which do "DFD" playback - rather than loading into RAM - you may wish to put them or just their wavetable data on the samples partition).
Samples are a different story.
Generally speaking, if you're rendering, you may be accessing both samples and wave audio data concurrently (depending on the type of audio project), so it helps if those two things and the render output partition are all on separate drives (ie. 3 drives total). To further complicate things, windows and some samplers tend to store some of their cache data in the pagefile (regardless of how much memory you have) and this is also read during sample-playback.
But your drive controller only has a certain amount of thoroughput - so, basically, if three drives are running at once, you may not see significantly more speed than two drives.
Further, if you're choosing between having three drives and two very fast drives, faster drives always win, regardless of configuration. So don't stress too much about partitioning - it makes the best of your existing hardware, that's all. You're better off with two 1tb 7200rpm's than 3 74gb 10krpms, usually.
(XP) Note for XP users in the modern age: If you're buying a new hard drive, most of the time nowadays it's going to be "Advanced format" ie. 4k sectors. This means you need to make sure all your partitions are aligned on 4k boundaries, since XP doesn't do this by default (Vista and Win7 do). To align your partitions, find out the brand of the drive you've bought and go to their website. All the major brands have alignment tools specifically for their drives. Alternatively you can download a partitioning tool like partedmagic, which will align them appropriately.
Hope this helps.
If you need help with doing the partitioning itself, I suggest you take a
look at the following articles:
A guide to managing partitions in windows (Administrative tools)
Disk partitioning explained (wikipedia)
Partitioning using BootItNG (shareware)