Sometimes I can just scroll by touching the screen with my finger. But sometimes that feature doesn't work and I can select text with my finger instead of scrolling. How do I switch between these "modes"? Is it just buggy or is there some feature?
I'm saying "buggy", and improved in Windows 8. For instance the Opera web browser just got updated in version 12.10: "We have added basic touch support for Opera in Windows 8 Classic, so you will be able to scroll, zoom and browse pages and elements easily." So you can scroll the new version around by touch - also in Windows 7. But not select text anymore.
That /may/ be better in Win 8 - apparently you can push selected text start and end markers around the screen like on an Android phone - and on Win 7, you can "enable the touch pointer". To do this, open "Control Panel", use the search for "Pen and touch", and the "Touch" tab of the dialog box.
What this should get you is an on-screen mouse that pops up for a second, anywhere you touch. Then you can for instance touch and drag the left mouse button to select text.
However, with the on-screen keyboard that I'm writing this with - Fitaly - I keep hitting this touch pointer mouse thing instead of the letters to be typed that are underneath it. This means that I need to type slower so that the touch pointer disappears before the second letter.
Another option is to use the stylus and/or flicks. You also might be able to adapt some "mouse gesture" program to work with touch gestures.