You might consider looking into partitioning. Partition your table so that the rows you want to get rid of are in their own partition then use SWITCH to put them into a whole different table, then drop that table. I've read about doing this several times although right now I can't think where. You should be able to find it easily enough. You might consider looking at Kimberly Tripps blog, she does a lot on partitioning.
The last half of this article seems to do what you are looking for.
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