A recent update released from Microsoft has caused Act! to crash when you try to use the documents tab.
For now it would be advisable to inform your IT of this and be selective with Windows updates. This issue is caused by updating Microsoft .NET Framework.
If you are experiencing the issue, take a look at this article http://kb.act.com/app/answers/detail/a_id/38261/, or send an email to support [at] evolutionmarketing.com.au and we will book a job to fix the error for you.