It is true that nokia 5130 is vulnerable to viruses, but not in the sense that the viruses will effect the phone. Viruses are .exe files or programs that can only harm a device like a personal computer(PC) which supports .exe files. S40 mobiles like 5130 can never support there files and hence are completly safe. The problem you are facing might be due to the following reasons,
1. Although .exe files can never harm this mobile or any other mobile of its kind, they usually take lots of space in the memory card. These files cannot be deleted as they are marked 'Read only' in the file properties, and to top it up they are also marked 'hidden'. So you can neither find it nor delete it.
2. This mobile supports themes. Whenever you apply a theme, a cache file of these themes remains in one of the system files of the mobile. These cache files will keep on increasing as as you keep on changing the themes. These system files are very well hidden and not even nokia pc suite can find these files.
The solutions to the above problems are as follows,
1. Download a java file explorer software from the net. This software can find hidden files and can also alter the properties of the files. The .exe files should be found manualy through the software and after finding the files remove the 'read only' property of the file. Then delete it. Do it for all .exe files. This will definately increase the memory. Since the software is a .jar file, so you can do this process in the mobile itself.
2. Use Oxycube, it is a software like nokia pc suite but more advanced. Go to download.com and get this software for free. This software can enter the system files. Hence making it easy to delete the cache files. The first problem can also be solved through this software.