I couldnot enter into my oracle XE.Can you please tell me how to enter

I have installed my oracle XE long before.but now i forget my username and password to get in.Its really want now.i have tried all my possibilities.that means check with different username and pwd.sometimes i got account locked, sometimes i got Account password expired,and sometimes its like Access denied by Application security check You must login with an account that has been granted the DBA database role. logout.but i am damn sure i given my username as example at first when i installed it.now that also not working .So is there any possiblitiy to find my username and pwd?or else reset the old one.Is there anyway to get in? or else should i deinstall and reinstall it again?