Bitcoin/blocks folder up to 17gb So I have downloaded Bitcoin-QT v0.8.6-beta.
It has downloaded everything and now this folder:

C:\Users\user\AppData\Roaming\Bitcoin\blocks


is 17GB

Anyway to prune/delete something?
The 17 GB of data in your blocks folder is the blockchain. This is the public ledger of all transactions in the Bitcoin network, from day one. A full node needs all this data to check each new block that arrives from the network. As time goes on, the size of the blockchain will only increase.

There exist other clients such as MultiBit which do not require the full blockchain download. These are called SPV or Simplified Payment Verification clients.