The following instructions should work for the installed version of electrum, but not for the portable version. The installable version of electrum puts the wallet files in a folder that's hidden by default in windows.
If you have a windows start button on your version of windows then click it, then copy and paste the line of text below into the search box. Afterwards press the "enter" key on your keyboard and the folder containing your electrum wallets should open.
%appdata%\Electrum\wallets
This is what the search box should look like after you paste the line of text into it.
Once your wallet folder has opened you can use secure delete software to delete your wallets if you want to make sure nobody can ever undelete them with special software. Do your own research to find some good software that can securely delete files by overwriting them with random data.
Never install anything recommended on bitcointalk before scanning it for viruses using
this site, and doing some research on the software yourself first.