You can the steps mentioned in the link below to reset the SA password:

http://blogs.msdn.com/b/raulga/archive/2007/07/12/disaster-recovery-what-to-do-when-the-sa-account-password-is-lost-in-sql-server-2005.aspx

> 1. Open the Configuration Manager tool from the "SQL Server 2005| Configuration" menu
> 2. Stop the SQL Server Instance you need to recover
> 3. Navigate to the “Advanced” tab, and in the Properties text box add “;–m” to the end of the list in the “Startup parameters” option
> 4. Click the “OK” button and restart the SQL Server Instance
> 5. After the SQL Server Instance starts in single-user mode, the Windows Administrator account is able to connect to SQL Server using
> the sqlcmd utility using Windows authentication. You can use
> Transact-SQL commands such as "sp_addsrvrolemember" to add an existing
> login (or a newly created one) to the sysadmin server role.
> 
The following example adds the account "Buck" in the "CONTOSO" domain to the SQL Server "sysadmin" role:
>
>`EXEC sp_addsrvrolemember 'CONTOSO\Buck', 'sysadmin';`
>
> Once the sysadmin access has been recovered, remove the “;-m” from the startup parameters using the Configuration Manager and restart the SQL Server Instance
>
> **NOTE:** make sure there is no space between “;” and “-m”, the registry
    parameter parser is sensitive to such typos. You should see an entry
    in the SQL Server ERRORLOG file that says “SQL Server started in
    single-user mode.”