A "system restore" does not delete any files.
All a system restore does is that it restores the system registry to an earlier date.
If the virus relies on the system registry to execute it at startup then this will "defeat" the virus because nothing is executing it at startup.
However if the virus is more sophisticated then that (most have been for the last 5 years) then a system restore wont do much.
This is why you need a real backup solution, preferably one for files and another that does full-disk backups. At which point you could easily restore your entire hard drive to its pre-virus point and then just add back in the files you need.