What computer operating system? Win 7(64)
What software for scanning and the settings used? - Canons own utility comming w the scanner.
What are you scanning and the type of media it is on? - B/W and col. neg:s mainly.
What are the bad results? Describe and upload an image. - Over all bad sharpness regardless the scanning resolution. 4800 dpi gives not better images than 1200.
Has the scanner ever given you good results?
- No really. I think the scanner is not acually suitable for scanning negatives or slides.
When did this change? Anything else change around that time? - I have made a few comparative tests with my old EPSON Perfection3200 Photo and it gives excellent results. I will probably go back to it.
I Googled "CanoScan 9000F poor sharpness" and read through a number of links. Seems you are not the only one with a sharpness issue with that scanner. Many opinions and a few things to try, like turning off everything but unsharp mask, use some sort of fluid between the negative and the platen glass, and even shooting your negatives with a digital camera.
The above images came from a thread that I found a fellow Swedish person - blåland - addressing your issue. See his post about 3/4 down the page here.
Here is a page that discusses your scanner and settings. Maybe his settings will help you.
Here's a link where a guy says (post #6) that Nikon software doesn't play nice with Windows 7. This was back in 2013 and may no longer be valid, if it ever was.
Some have suggested a different scanning software, like VueScan, though if the scanner is technically deficient then even the best software will have a tough time overcoming certain deficits without introducing other artifacts.
These are from just the first few links on Google, but the issue seems a common complaint. Check out Google as there may be more info since I only read through the first six links.
Unfortunately, if you cannot find a reasonably solution then it's back to your Epson scanner or shell out more money for a better dedicated film scanner.