Try Right-clicking on one of your pictures. Select Open With, and "Choose Program".
Look for "Windows Picture And Fax Viewer"
If it is there, select it. Then check off "Always use the selected program to open this kind of file". Your pictures should now open in the Picture viewer, which allows you zoom in/out.
if it doesn't appear in the list, though:
1. Start->Run. Type regsvr32 shimgvw.dll
you should get a message, DLLRegisterServer in "shimgvw.dll" suceeded.
Now see if it appears in the Open with list. (however, I might add that usually it also re-associated itself with jpegs and gifs and so forth, so you might not need to perform the open with stuff I described above if you need to re-register the dll)