Microsoft Vista Home Premium is currently installed on My Acer Aspire 5310 laptop, as came with it as standard. Everytime I open the programme, I manage to exit the registration window, and avoid it.

Today, everything turns grey, and it displays a message at the bottom saying that the functions are locked.

You have Vista installed which is the operating system and quite separate from Word which is the word processing package which you seem to have been given the trial version of. You say you manage to exit the registration window - why? If you want to use it, you need to unlock it by registering it.