account not up to date!

Please people be advised to go over your account statements . We bought a laptop from hificorp 3 years ago! The contract was for 2 years. We skipped 2 months and we payed that instalments aswell! 3 years later and they still say we owe R4500 on a laptop that only cost us R6500! But they have been takeing R600 monthly of our bank account? How is that possible. They phone me easily 5 times a day. Isint that harrasment aswell? Even though we're up to date an past being up to date. This is not the first time we have had a problem with this spesific company!