I generally use the directory where the program is stored in.  You should always delete the file after use, and when creating, check to see if the file exists and delete.

If that doesn't suit you, you can use the Windows temp directory.  Use the GetTempPath() API function to get the Windows temp folder.  This is much better than using C:\temp
If you dunno how to call API functions (I haven't used .NET so I can't help you unfortunately), it's probably a good idea to learn how (API functions are useful).  Microsoft's MSDN will outline the structure of API functions if you need a reference.