Your hardware is fine. It's just that scanning slides is tricky. Not impossible, but tricky. What I find most difficult is that some beautiful colors you see on the light table are almost impossible to scan... it all come down to the challenge of translating an illuminated color to a bitmap color.
One thing you may want to check out is the program VueScan (www.hamrick.com
). Demo it first before you buy. Vuescan is a swiss army knife of scanning software. It cracks open your scanner and gives you access to its nuts and bolts. It is not a simple tool to use, but it can be a very very effective one (I prefer it to the Minolta software that came with my scanner).
A couple sites to check:General scanning tipsColor neg scanning
(I know, you are doing slides, but he give a good description of VueScan).