If it was made after 2003, then it will have strong AES-256 encryption which is bad news. If its older than that you can crack it in a about an hour i think.
You can either brute force it or dictionary attack it.
If you think the password may have been a weak dictionary based word then a dictionary attack would be your best option, and should be fairly quick.
If it was a secure password, brute force is your way to go. Problem is if the password is greater than 7 lower-case characters then its not gonna be crackable on your home PC. A 7 letter lower case password will take a good few days to crack, if you add numbers or upper case in then this time rockets upwards to months/years.
Guess it depends on how badly you need the data? The best software for the job IMO is
http://www.elcomsoft.com/archpr.html £50 price tag but its worth it, includes multiple vulerabilities which may work instantly instead of having to brute force the password.
There is no doubt free options around, don't know any off the top of my head though but google will
