create table permission denied in database 'master'

I have installed the free version of sql server 2008 (sql server management studio express edition) on my PC. After installation I get the following error

create table permission denied in database 'master'

I tried reinstalling several times, but I keep getting the same error. When i checked

select user_account();

It showed that I was logged in as guest. How do I solve this? since I am not permitted to create a new login.