Lets do the maths!
If you open up character map in XP, click '!', then in the bottom half of the screen it shows the key code as '0x21'.
If you then find '¡' the character code it shows the ansi code 0xA1 and the alt-XXXX code as 161. Add these together:
0x21 + 0xA1 + 161
and you get 355.
Sorry guys, I never was quite as good as Carol Vordaman :(