As most of todays virus scanners does heuristic and scan string search through files, they would be rendered useless... as they'd consider a file with 100% readable characters as a text file or perhaps as a script... but they would not try to decode it unless it's obvious that it is UUEncoded. Even if they did, there could be an encrypted layer which scanners would not recognize as an executable.
... and it wouldn't be that hard to hand craft together a decoder that would run under Win32...