Are you sure you don't need a new scanner? Flatbed scanners scan photographs and documents using reflected light. Scanning negatives requires the use of a scanner which can recognise images via transmitted light. (i.e. the light has to pass through the negative, rather than being bounced off its surface).
Some flatbed scanners do include the facility to scan negatives. (i.e. they come with an appropriate adapter). Such 'dual-role' scanners used to be pathetic at scanning negatives (because of their low image resolutions) but some newer flatbed scanners are now just as good as many proper film scanners.
If your scanner has the facility to scan negatives (by passing light through them, rather than reflecting light off them), the printer driver should give you the option to change the source to 'negative film'. Simply using that option will automatically reverse the colours, so that a positive image is imported into your image manipulation software. (i.e. you don't need any additional software).
If your scanner can't handle scanning by transmitted light, PC World were right and you do need a new scanner.
Chris