1. I use a computer everyday
 
    I use computer programs to prepare teaching materials.
 
2.  I play the piano everyday
     I have another job - I play piano in a band every weekend. 
 
in this instance you can use either �piano� or �the piano�, as in every case I can think of while writing this post...
 
 
3. play the �cello or play �cello  ( �cello is short for violincello) - this is the same as for 2.
 
I play �cello in a youth orchestra but violin at school
I play the �cello because I love the way it sounds.
 
 
For 2 & 3 �the � would generally inmply a specific instrument whereas the bare instrument name implies a musical instrument of that type/family.
 
Cheers,
AB