Q1) "I have a charge off from march of 1999. I am having big problems with credit because of that. Is there anything I can do to make that go away?"
A1) Assuming that the 'unpaid debt' (there is really no such thing as a 'charge off') was valid, there is no way to get it removed. (That would kind of defeat the entire purpose of reporting your not paying, wouldn't it???). If the report is NOT valid, you can ask that it be investigated and the credit agency will remove it IF it is incorrect.
Q2) "And if not after 7 years does it come off my report even if I never pay it?"
A2) The negative report will stay for 7 years, then is supposed to 'drop off'. However, it is not unlikely for it to remain until YOU ask that it be investigated and removed.
Q3) "What will happen if a collection agency buys it and demands payment and I don't pay them?"
A3) Usually nothing. They will harass you (and usually will lie to you about what they will do), but in reality they are pretty harmless. To find your rights, read the FDCPA at http:www.ftc.gov/.
Q4) "Will that add another charge off to my credit history?"
A4) Only if you 'refresh' the stature clock by making a payment.