How to prevent SQL transaction full

Hi all,

Here's the story.

vCenter 5.0, SQL express 2005.

Last week, I changed the settings of Log history in vCenter from unlimited to keep 30 days.

Then after few days vCenter crashed because the SQL transaction log was full (cap at 5GB). So, I follow the KB:

- SQL restore: Simple

- Shrink the log file.

But today, after few days, the transaction log is full again.

How do I prevent the transaction log from becoming full?

Thanks in advance