if i already have MS office but its trial version how can i download the free full version?

There are no free MS Office versions available for download.

You should rather have a look at SSuite Office for a free office suite. Their software also don't need to run on Java or .NET, like so many open source office suites, so it makes their software very small and efficient.

You may try these links for more information:

http://www.ssuitesoft.com/index.htm

or

http://www.ssuitesoft.com/ssuiteexcalibur.htm

or

http://ssuite5element.webs.com/thefifthelement.htm

You don't. The trial is already used up, it keeps tabs of much time you have used it as a trial. Even if you have uninstalled and reinstalled it. You options are to BUY microsoft office or use some other alternative. (such as OpenOffice or Google Docs) Microsoft does not give its microsoft office programs for free, so giving links to pirated software is considered illegal.

There isn't, nor has there ever been, a free version of Microsoft Office ... unless you're not bothered about being a thief.

You could try OpenOffice.org, which is a free Office Suite, that is compatible with Microsoft Office documents, databases, spreadsheets, etc.