You have made the common mistake of confusing the TiVo, a computer-ish purpose-dedicated appliance, with an actual computer.
Almost any attempt to use a TiVo as anything other than a TiVo is cost prohibitive at best.
When you say TiVo HD, you mean a TCD652160, correct?
Without a subscription you might be able to use it as an OTA and cable tuner that won't record or play back, except for the 30 minute buffer, and puts up nag screens about the lack of a subscription.
How long have you had it and how long has it been without a subscription and do you have any other TiVos currently in service?
(I'm trying to figure out if by some very lucky chance you might be able to get the $99 lifetime subscription offer on it)