I doubt it is a virus, svchost.exe is just a system process that multiple types of windows services use.
System idle is actually not using 50% , it means 50% is unused.
Now as you know the more you have running the slower things will be. You could have services running that you do not need to be running and we can disable those.
Depending on how much ram you have the amount of things running could be too much.
Here are some things you can do to speed this up.
First Disable Automatic Start Up Programs:
Click Start and in the search box type msconfig and press enter.
Click the startup tab at the top and deselect all of the programs that are listed and starting automatically.
You should be fine disabling them all however if you see anything related to your wireless adapter(if you have one) leave that checked. You can easily re-enable these if you need to.
Many programs automatically add themselves to automatic start up slowing down the machine and you do not need them to.
Next do a good malware scan:
Please download this free program by clicking on the following link. On the page click download latest version on the top right.http://www.filehippo.com/download_malwarebytes_anti_malware/
Start the installation, you can uncheck the start trial button as you do not need the paid version, the free one is fine. Once installed, open the program and do a full scan (This may take up to an hour or longer in some cases). After it's done, press ok, then click the show results button in the lower right.
On the next screen, check all the boxes and then press the remove selected button to remove any infections that it found.
Dooing the above should help, you may have some software installed that is using a lot of resources and it is possible you could have some infections that could be easting system resources but try the above and we will go from there.