Do you use Word, if so you should find there will be a invoice template for creating your own. If you open one, you should be able to edit it to suit your self then save it with a new name so it's your own personal invoice.
You can check if you have template go into Windows Explorer look for a folder that say's MsOffice or Office click on the crosses at the side of folder till you see a folder called templates if there is one then look at the files for one that looks like it mite be an Invoice template it may only use the first letters double click on it and it should load word
If you look in templates folder as woodchip suggests, it is in a folder called spreadsheet solutions (in my version of office 97) invoice is then an excel file, double click to open, if you click on the grey box (near top right hand corner once opened) called customize then you can add yours and customers info as required. Template can be saved as a blank/master once your info has been added.