BASIC programs on the PET & VIC20 start at load address $0401, on the C64 they start at $0801. The C64 is able to load a PET program correctly (Commodore doing a bit of forward thinking here), but not the other way around, the program will load as garbafe.
This will fix it :
1) Type NEW
2) LOAD the C64 program
3) Add a new line - 0 REM
4) POKE 1026,4
5) Delete line 0
6) Type LIST - your program will be there!