Microsoft is releasing a preview version of Windows 7 RC (release candidate) which you can use until it expires in June 2010. That means about 13 months free usage from current date (May 2008).
PC World reported that this is the longest trial usage ever given by Microsoft. The Vista trial usage was given about 7 months usage from release till its expiry in June 2007. Once it expired Windows Vista will auto reboot every two hours to remind users to get a paid copy. I guess Windows 7 will be of a similar vain.
Anyway 13 months is quite long. And for some of us, we do reformat the hard disk every year or so, and 13 months seem like a good duration. In fact I was already thinking of reformatting my hard drive so it might be a good time to give Windows 7 RC a try. Only problem now, Microsoft server is swamped from too much download request. Guess I will wait awhile before going for a download.
PC World article reference:
Use Windows 7 free (for awhile)