If you are looking to enter special ASCII characters via the keyboard, for example into a Notepad or OUTLOOK mail session, then you should be able to hold down the Alt key and at the same time enter the ASCII code in decimal number format that represents the special character via the NUMBER KEYPAD, then release the Alt key and the special symbol should appear on screen. Note that it does not work for all applications. It should work in a system Command window. Note the numbers MUST be entered via the active NUM KEYPAD, else it will not work!
The code 00A3 that you say represents the pound symbol, is a HEXADECIMAL code which translates to 163 in decimal number format and does not represent the ASCII code for the Pound symbol! It should be 156 decimal number i.e. Alt+156 on the NUMBER KEYPAD.
The Code 00A3 Hex=163 Dec which produces a special character of ú.
A list of all ASCII codes can be found at
http://www.asciitable.com. The Dec column is the code numbers to use with the Alt+NUMBER KEYPAD sequence.