Your phone is indeed one of those models, for which viruses are relatively widespread. Emphasis on "relatively". You're still much, much less likely to encounter a virus for your mobile phone than a virus for your PC.
One advisable precaution is to set up Bluetooth in non-discoverable mode and not to accept (i.e., delete them without viewing them) MMS messages (SMS messages are fine) - even ones coming from people you know. Also, don't put MMC cards from other phones in yours, since this, too, is an infection vector. However, given the specifics of your model (the MMC card is difficult to access), you're unlikely to do that anyway.
If your phone becomes infected, it is much better to use an anti-virus program that knows how to remove the virus. Still, if you insist on formatting your phone - that can be done too. Just keep in mind that formatting will remove the virus only from the main memory of the phone. If it has infected also the MMC card, it will immediately re-infect the phone from there. So, make sure you remove the card before formatting the phone. Format the card (or just delete all files from it) on a PC with a card reader - do not attempt to format it on the phone.
In order to format the phone, do the following:
1. Make sure the battery is fully charged.
2. Turn phone off and remove the MMC card.
3. Now this can be little tricky, but it's not so difficult. While the phone is complelty switched off, you must hold down three keys. These keys are the green call key, the star (*) key and the number 3def key. (If you hold phone laying in front of you on a hard surface it makes the procedure a lot less tricky.)
4. Once you have all the three keys firmly pressed, you need to reach out with your right index finger to switch the phone on. You must not release any of the 3 buttons while doing this, or the formatting will fail.
5. Keep all the three keys pressed untill the phone asks you to enter your country, then date and time. That's when you know the procedure has been succesfull and phone has been fully formatted. If you do not get prompted to enter the country and date and time, then the formatting has failed and you must try again. It's likely that you have released a button while switching the phone on. If it doesn't work on the first attempt, keep trying.
There are also two other ways to "format" the phone which are less tricky to perform but which delete less, so a virus might survive them:
1. Restore original phone settings: enter the number *#7780#
2. Soft format: enter the number *#7370#