http://www.tedmontgomery.com/tutorial/ALTchrc.html (google alt char codes)
Note - also depends whether the character in question is included in the charset within the default font for the site in which it's used... so where alt-blahdewhateva may result in a copyright symbol here in the verdana fontset, go to a site that uses Tahoma fontset and it may just show a square box... if using these codes, best to stick to standard typefaces - arial, verdana, helvetica, times new roman etc etc.
Also, US & UK Keyboard layout can result in differences.
Greek symbols as you're referring to are done as ampersands under html - more a html code than a windows alt-char code...
http://www.tedmontgomery.com/tutorial/HTMLchrc.html
or via the builtin windows "symbols" fontset... requiring the font to be defined first, but won't work here as inline styles are used and it won't parse <font> html tags correctly (i think)
http://www.tedmontgomery.com/tutorial/SYMBchrc.html