Can someone help me out here, i mined from the beginning day one, but stopped mining years ago. Never looked at my coins again and kept them on my PC. Now i cannot sync the wallet. I need a new one, can i backup the DAT file, install a new wallet and put the old dat file back?
You could backup the Wallet .dat, install the new Wallet on top and let it sync. It´s not necessary to re-import the .dat file.
Yeah, as hallo_frosch said, just install the latest wallet & copy paste your backed-up wallet.dat over the newly installed wallet file. Then the wallet will need to resync the chain with the new wallet file. It will take several hours probably but just leave it overnight and you'll get your coins back fine. Make sure to backup your wallet file before you go copy-pasting, just in case.