Can I return a PC after opening

Yes u can with in 14 days

Yeah as long as its less than 14 days since you bought it

I would imagine yes, as long as you have the receipt.

Amm is good the Store ?

Within the normal Best Buy return policy, yes.

Yes once you have the receipt

yes you can

Yes as long as as it is within the retirn window. No box is even needed!