This encoding is not dependant on the machine it is done on. That's all there is to it. So in order to write a decoder one needs to find the pattern and the character combinations. If one encoded this, one basically knows the 3 letters a maps to. I couldn't find any obvious connection between the the input character and the output, hence the table.
Next step is to extract the pattern.
This is also very simple by encoding the same file as above with a lot of a's, and then have a script dump this as an array to include in the perl code. With this information one can write a perl script, like the one below, which will decode the asp file. I've tried this on some real asp code as well. But I'm not really a windows person. I searched the ms site, but couldn't find anything.