hmm, There's two ways you can go about this. The shortest way is...
Install windows on the sata. When windows is all up just go to "My Computer" on your start menu, right click it and select Manage. Click on Storage in the right window pane, then select Disk Management. Here you will see your sata drive, as E. Right click it and select "Change drive letter and paths" and see if it'll let you change the lettering.
Second method is to see if you have a version of windows installed on that old IDE drive (either another XP, or something older)? This may be causing your lettering conflict cause if you do have a windows installed on that drive, it already has the C: drive letter taken. I'm assuming the newer sata drive is clean and has nothing installed on it (especially another verson if windows cause if it does it is also causing the lettering conflict). Here's what i'd do. Disconnect all the connections you have connected to the IDE drive, from the IDE cable itself (so the mobo won't detect it) to the power cable. Only have your sata drive connected and powered. You can still have the IDE physically in the case but just dont hook it up. Boot up your PC and if this sata drive is new and has not been formated yet windows should, by it's own default, assign windows to be installed on C: instead of E:.