I would suggest moving to Windows xp, I think the only difference of 64 bit and 32 bit is that 64 bit allows you to see more that 3GB of ram where 32bit can just see upto 3GB of ram not allowing you to have more.
What are your system specs?? a more detailed post would be nice.
Also to answer your question, I think you would have to format your C:\ and then install the new OS. so better to backup everything u have on C:\ onto an external drive and then install the new OS