In Exchange 2010, you can use Retention Policies to manage message retention. Retention Policies consist of delete tags, i.e. retention tags with either Delete and Allow Recovery or Permanently Delete actions, or archive tags, i.e. retention tags with the Move To Archive action, which move items to the user's archive mailbox.

Depending on how they're applied to mailbox items, retention tags are categorized as the following three types:

1. Default Policy Tags (DPTs), which apply to untagged items in the mailbox – untagged items being items that don't have a retention tag applied directly or by inheritance from parent folder. You can create three types of DPTs: an archive DPT, a delete DPT and a DPT for voicemail messages.
2. Retention Policy Tags (RPTs), which are retention tags with a delete action, created for default folders such as Inbox and Deleted Items. Not all default folders are supported. You can find a table showing the default folders supported for RPTs in Understanding Retention Tags and Retention Policies. Notably, Calendar, Tasks and Contacts folders aren't supported.
3. Personal Tags, which are retention tags that users can apply to items and folders in Outlook 2010 and Outlook Web App. Personal tags can either be delete tags or archive tags. They're surfaced in Outlook 2010 and OWA as Retention policies and Archive policies.

Read the complete blog at http://blogs.technet.com/b/exchange/archive/2011/08/05/prevent-archiving-of-items-in-a-default-folder-in-exchange-2010.aspx

Read my favorites blogs:

Designing a backup less Exchange 2010 Architecture

Step by step guide for upgrading Active Directory from Microsoft Windows 2003 to Microsoft Windows Server 2008

Microsoft Exchange 2010 CAS Array – Steps and Recommendations

Appear Offline in Microsoft Office Communicator Server 2007

Microsoft Exchange 2010 Test cases

Microsoft Exchange Server 2010 Disaster Recovery