Sim cards well at least older one hold 10 messages and 100 contacts.
Make sure:
You have deleted sent messages as well as your inbox and also read reciepts if you ask for them. Also make sure your MMC has not tried to copy its contents to the SIM as this often happens.
The Sims are pretty hardy and viruses are rare and do not effect the SIMs often rather the phone itself and its operating system especially Symbian menu phones.
Let me know if this helps
