The official rules on using the Euro symbol € are at http://www.cs.tut.fi/~jkorpela/html/euro.html.

Windows has big problems with the Euro symbol. Out of the box, many Windows installations, including nearly all in the US, set the keyboard to English (United States), which cannot display the Euro sign. Microsoft claims that setting the keyboard layout to US Internatonal solves the problem. It does not. Microsoft publishes two fixes at http://support.microsoft.com/kb/306560, one do-it-yourself, the other automatic (which naturally requires Internet Explorer). Both fixes install the US International keyboard layout, and neither fix allows display of the Euro symbol.

In HTML you can represent the Euro symbol as € or as €. Given the problems Windows has with the Euro symbol, if there is any chance that your data will be displayed on a Windows system, it's best to use the officially recommended abbreviation "EUR".

Many recommend using ISO abbreviations for all currencies. The symbol for UK sterling is easily confused with the US 'pound-key', which outside the US is called 'hash'. Australian, Canadian and US dollars are easily confused, and the UK sterling symbol is also flakey on Windows machines.