Can synchronization be interrupted and continued later? I was using the official Bitcoin client some time ago.
It was version 0.5 I think. Then my computer crashed and didn't use Bitcoin for a long time.

I was lucky because I had a backup of the wallet.
Today, I installed the latest version and it's different.
It is running for many hours now but it's still at 18-20 %.

Should I leave it as it is or I can turn off my PC and continue tomorrow?

If I use my old wallet, will it start from the begining?
If yes, is it better to stop the software, 
delete everything, replace wallet and start all over again?
It takes a long time (maybe a couple of days) to download the entire blockchain. You can interrupt it at any time and it will continue where it left off when you restart the Bitcoin client.

The blockchain (which is the public transaction ledger) is independent from your wallet (which is your private keys). You can load your old wallet into the Bitcoin client at any time.