However if you have sensitive stuff on there like work-related info which involves confidential company dealings or secrets, or if you work for the military with secret/confidential info, or have confidential client information on there, etc. then yeah you might want to be more careful. Perhaps If you have sensitive personal financial information on there as well, you might want to be slightly more careful. Why you'd have any of that on an HTPC is beyond me but anyway, in that case there are lots of programs on the web you can use to securely wipe an HDD, writing 1/0s, etc with multiple passes.
If these machine(s) have SSDs there are other, much easier ways which give pretty good results (and really you wouldn't want to continually write/erase an SSD 'cause you're going to risk drive failure and lower the drive life regardless).
If you're really that worried, you could always just sell said computers without the hard drives, and take the loss in $$ it would result it. Or you could buy new HDDs and just fresh-install on those.
If it's a standard hard drive, I'd wipe the drive and perform several low level formats to ensure complete erasure of any existing data.
If you've got actual banking info stored on the computer, like spreadsheets, access information, identity information, etc. etc. then yeah you might want to be a little more aggressive in your effort if you're that worried about it. Otherwise I'd just format and re-install and call it a day. You can also do that and then use a program to just wipe the free space at the end of the drive which won't be so time consuming.
On an SSD, as said, you don't want to start writing data and low-level formatting, etc. Just do a "secure erase" (maybe more than once if you feel you need to) with software provided by the drive maker, and I think you should be fine.
Keep in mind that data recovery, even on an HDD is pretty darn hard to do for the regular computer user and the vast majority of the population. Just ask people that have unintentionally lost data who have tried to recover it, lol.
For your spinning hard drives, my go to wipe tool is DBAN. This will boot into an environment that will give you a number of options for wiping the drive. A single pass of zeros is sufficient to protect you from everything short of a state sponsored attack (and there is some debate around their ability to read drives after a single pass). Anyone that wants your data that bad is going to get it anyway.
Advice above regarding your SSD is accurate.
What are my options now?