You can use various utilities to create and change partitions... Lets say for example you have a 500 gig hard drive in your system... You can create a 100 gig partition for your main drive (C) and a 400 gig partition for your data drive (d, e,) ... This way, if you ever have trouble in the future, you just reformat the main partition... Once reformatted, all data on the second partition will remain intact given you didn't have a virus or spyware that compromised the data...
There is a utility when you boot from the windows cd to reinstall the OS...