Firstly, if you can continue to get your machine booting, backup all your documents onto CD or DVD if you can.
Imaging a drive onto another drive can be difficult, it all depends on how much experience with hardware you have. As always, if you don't feel comfortable doing any of this take it to a repair shop or knowledgeable friend and ask them to do it.
Imaging a drive is the process of making an exact copy of everything on your current disk and transferring it to another disk (one that isn't faulty). To do this, ideally, both hard drives need to be in the same machine at the same time. You then run some software (Norton Ghost or Acronis true image backup for example) to copy the old onto the new.
When this is done you can remove the old disk and continue using the new one as if it was the old one. Make any sense? (didn't think so

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Both the above mentioned software are commercial, but I believe there are some similar freeware products out there.
As you are having problems, this solution will only resolve the issues if the data copied from the faulty drive is not corrupt. Again - make document backups first, if this does not work you will probably have to re-install windows and all your applications.