[Crizz]

If you´re new then probably you have no mod -> a custom modification yet installed to it.

So you shouldn´t have a prob with your Single Play. DLing and installing a custom map can cause a conflict, if it has a modified .tik file in it, which shouldn´t be included directly into a map file!

Usually custom .pk3s are named User-xxxxx.pk3 (xxxxx is for the mod name) so it´s easier to find them in your main folder.
The regular files are named pak0.pk3, pak1.pk3 and so on. Never modify or delete anything from these files.
Custom mods replace one or more files from the original game, when installed. To have the old FX simply delete the mod you´ve installed before.

I hope I wasn´t confusing :-)