A sliding window partitioning strategy could help here.

Dan Guzman has a couple of nice posts on the subject:

http://weblogs.sqlteam.com/DANG/archive/2008/08/30/Sliding-Window-Table-Partitioning.aspx

http://weblogs.sqlteam.com/dang/archive/2008/09.aspx

Setting up the partition scheme and rebuilding your indexes on the partition scheme can be done online if you have Enterprise Edition.

Hope this helps

Gianluca