As one who battles with the wretched machines every day, I would prefer to abuse them physically instead of verbally. But the authorities and the machines' owners frown upon that... ::sigh::
Caveat: I must caution you that I am a mere beginner, and recommend that you have this verified/corrected by a speaker with more knowledge and experience before committing yourself to it.
To give you some qualified answers:
"You stupid computer"
{De'wI' QIp}
"Worthless piece of junk"
lo'laHbe' -- v. worthless
veQ -- garbage
'ay' -- component, part
{lo'laHbe' veQ' ay'} or {lo'laHbe'veQ'ay'} (I can see arguments for both the use of separate words and one longer compound word. Opinions?)
The equivalent of the word "crap" may not have the loaded meaning in tlhIngan that it does in English; I do not know if a literal translation will have the effect you desire. (Then again, the phrases you listed above do not come across to me as having a particularly corrosive effect on young ears.) Two approximations would be:
porgh -- body
DI -- litter, rubble, debris
puch -- toilet
{porgh DI}, or perhaps {puch DI}
I would personally prefer to omit the word "piece". The only word we have for it, {'ay'}, implies something that is part of a larger, working whole.
In tlhIngan Hol, the best swearing comes not so much from the most offensive words, but with the composition of the most creative insults. See the thread on
Curse warfare for examples and inspiration. If you come up with any particularly good invective against those piles of mineral and plastic that threaten to take over our lives, I encourage you to post it!
-=- Kesvirit