if I have bitcoins in my desktop wallet and I'm needing format the hd, I'll lost the bitcoins after format the HD?
You won't lose it in three cases:
1. If you are using a wallet that give you a 12 words passphrase (e.g Electrum) and you write them down somewhere safe, even If you format your hard drive, you can restore the wallet afterwards with that passphrase.
2. If you exported the private keys for the addresses that contains funds and printed them (or simply kept them safe on a USB stick), you can then import them back later.
3. You have the backup of wallet.dat and you know the password (If its encrypted)