Like buying anything else on credit a 0% offer usually means paying a higher price for the product, or unrealistic delivery charge or a short payback time. There are very few Free Luches in this world One really has to sit down with blunt pencil and back of envelope and do the sums.
Some companies will let you pay a deposit, then give you 6 months interest free credit provided you pay off the balance.
If you proceed with this method, then the matter is transferred to a finance company. They will give you a choice of payments if you cannot pay the balance.
An alternative method is pay the full amount on your Credit Card, then pay back the Card company as and when you can. This is probably a more flexible method, especially if they give an interest free period on new purchases.
If you're going to do it with a credit card, get a new one with a 0 per cent introductory period and buy the PC with that then pay off the balance before the 0 per cent period is up. Borrowing on a credit card at their standard APRs is an expensive way to finance anything.