1. Go to folder options (On win98 it's View->Folder Options in Windows Explorer and on other windowses it's Tools->Folder Options).

2. Go to file types
3. Try to find the .sit file. If it's in the list:
a) If you use XP then click Change... Then "Select the program...", and select your program.
If you use other version then click edit... select the action in bold, click edit...
b) Select your new program