A clean install will only affect your C drive, format it to get best results. The installation procedure will not affect your data drives unless you tell it to... If you are still worried disconnect your data drives until you've installed windows!
Before you initiate setup go to Computer and make a note of the size of the drives, especially 'C'. When you get to the installation screen the drives are listed as partitions
not by their letters, and we don't want to install to the wrong one...
When you're ready boot from your Installation disk, choose Language etc., 'Install Now' and 'Custom' (Advanced).
Next screen is 'Where do you want to Install Windows'.
You should see Partition 1 'System Reserved' Partition 2 'Primary' (Should correspond in size to your 'C'drive) and then Partition 3 etcetera.
Select partition 2 and 'Advanced Drive options'
Choose 'Format' and then Install to that drive. Your logical drives should not be touched, but it is always best to backup your really important stuff to an external drive if you can.
Do you even have 4GB+ of RAM? Do you want to use applications that will benefit from 4GB+ of RAM? If not, then I seriously wouldn't bother. You will gain literally NOTHING "upgrading" to a 64bit OS... except that it will use more disk space and potentially unavailable drivers for old hardware.
I have 2 gb ram but will soon upgrade..frnd told me upgrading would increase performance..
64 bit OSes DO NOT have better performance than their 32 bit counterparts when both only have 2GB of RAM available running 32bit applications (which frankly is about 99% of apps). Any performance increase in any given application is due to the 64bit OS being able to utlilize more RAM - but only if you have 4GB or more. The fact is MOST apps only use 2GB anyway (because they are 32bit apps), so most users gain NOTHING performance-wise from switching to a 64bit OS.
That's the main advantage of 64bit OS, 32 bit can't handle greater than 4Gb RAM. BUT unless you have a specific reason for using greater than 4 Gb, the upgrade won't be worth the hassle. Do your upgrade to 4 Gb then keep an eye on your memory usage.