Welcome!
I'm not sure if that means Inkscape is being installed already, or some other program is being installed. Here's what I would do:
1 - Wait until the next time you start the computer. Then whatever was being installed should be finished.
2 - Check to make sure Inkscape isn't installed, in case that was the source of the message.
3 - Delete all the installation files you might have already downloaded.
4 - Download a fresh installation file. (If it's a laptop, you can use either 32- or 64-bit. If it's a desktop, you can only use 32-bit.))
5 - Try again to install.
If you're getting the message after you've allowed plenty of time for whatever other program might have been in progress, then it's probably something wrong with the computer. Note that some large programs might take 15 to 20 minutes to install - sometimes maybe more (although that's mostly from memories of Win95 or WinXP).
Let us know if you get any other kind of messages, or whatever happens. Good luck