You have just recently purchased SilverFast and your 35mm film scans come out pixelated? While this is still a common issue, especially if you're new to scanning and just dipping your toes into 35mm filmscans, there's an easy way to solve it in SilverFast.
When scanning 35mm filmstrips you want to make sure to choose a setting under "Format" in the "Scan Dimensions" widget of SilverFast, which looks like this (may vary silghtly depending on your SilverFast version):
When choosing a setting under "Format", like 4 x 6 inches for example, you will ensure SilverFast uses a higher input resolution to achieve a 300ppi output resolution in your desired output size. The scan dimensions widget will adjust the output resolution automatically with it's built-in resolution processor. In this example it calculated a 1600ppi input resolution for a 4x6 inches output size at 300ppi.
If there's no Format setting selected in the Scan Dimensions widget, SilverFast will assume you want to scan your 35mm slide at 100% and scan your 35mm with 300ppi as input and output resolution.
Check your scanframe in the preview area after choosing a setting under Format, since it will reflect the aspect ratio of the Format chosen ans a slight adjustment of your crop may be necessary.