Here goes:
1. "Welcome" on the login screen should be lowercase
2. Balloon is too high (should touch the icon), close icon is too small (should be roughly the same height as the balloon title)
3. About Me is missing the scrollbar on Firefox
4. Wrong gradient for "Social Links"
5. Start menu should have a shadow
6. In My Projects, two tiles are loading forever
7. Windows that cannot be maximized, but can be minimized, should have all three buttons, with the middle one disabled
8. Paint did not have the Windows logo in the corner. It would be better to show the JSPaint menu bar to make things like Undo accessible, and the JSPaint authors deserve attribution.
9. "Git Co-pilot" is not a thing, as Git ≠ GitHub. (On the XP project page.)
If I were making something like this, I would probably skip the boot and login screens (certainly would not require user interaction; indeed, XP would automatically log you in if you had a single passwordless user), and show "About Me" on startup, so that potential clients don’t give up before they learn more about you.