Gimp works fine with nearly every image format (except CorelDraw),
but for just viewing you can either try and find some simple'n'cheap version of ACDSee at the stores (like I did) or get
IrfanView. I don't know for sure wether it works or not, but Targas are a quit popular format so it should do so. The program handling is (afaik) mostly the same as with ACDSee, so there shouldn't be any problems with using it (like there are with gimp)