Hi j_unaid,
some questions for you
1) Who told you that the bank would provide a guarantee?
2) Was the guarantee to do with you receiving the computer? Or was it to do with the seller receiving the money?
3) How did you arrange the purchase?
4) Was it a private individual you were buying the computer from?
5) Did you transfer the funds to the sellers account?
I think unfortunately you have been ripped off, or more bluntly - robbed. Banks won't usually hold a straightforward transfer to a personal account insofar as I know and unless you arranged it with the banks in question, I doubt very much so that it would have happened.