With our impending upgrade to vB3, we are planning to prune all PMs older than 30 days, and to also prune the attachments on a yet-to-be-determined criteria. This is to make the upgrade go faster as well as to help increase the speed of the board slightly. You should find any old PMs or attachments that you want, and save them on your HD. Other tables will be pruned at the same time, but the effects should be less noticable.