I have a couple of bitcoins but my older computer crashed. But I had a backup of my wallet. Months passed and one day I decided to download the official client and sell my Bitcoins. But when I went to bitcoin site I saw that the client was different and very heavy. The site recommend to install Multibit. I got Multibit, but found out that I couldn't import the old wallet.dat, they wanted a different format. So, I'm stuck now.

(I also installed Bitcoin Core today, and it running all day and still is at around 10%).

Any solution?
Can I put the old wallet at the official bitcoin client?
Somewhere I was reading that all you have to do is place the old wallet here:
C:\Documents and Settings\YourUserName\Application data\Bitcoin


I also found this: blockchain.info/wallet/import-wallet

but I'm not sure if it's safe and how it works.

Can somebody please help me?