Hi,
What I'd suggest as a first step is to register with The Motley Fool, UK (www.fool.co.uk), and then ask for help in cleaning your machine via this forum "Computer, Help With This Blasted". There are some very knowledgeable people on that board. It will probably mean doing something like getting into Safe mode, then downloading and running Malwarebytes or similar to clean your computer. Once cleaned, you will be advised to use a good anti-virus and firewall of which there a few free versions available for home use.
Don't be put off by the fact that this is a financial site, just navigate via Boards, Boards Help & Guides, Boards A-Z and then step down the listing to Computer, Help With This Blasted where you will be able to post your question after you have registered. You will need pseudonym to register, and when posting your question, it will be helpful to provide details such as computer type, make, etc, Windows version, any anti-virus etc plus what is happening.
I would also suggest that once clean, you should perhaps try downloading SketchUp from a different site. I don't know where, but I'm reasonably sure there will be one somewhere.
I assume, by the way, that you are using a PC and a version of Windows.
Best of luck,
Peter G. Shaw