The first thing I would do before doing anything else is back up the wallet.dat file to at least one other place.
Probably the most straightforward way of accessing this wallet and spending any coins inside is to simply download, verify, install Bitcoin Core, and let it sync. Once you've done that, replace the wallet.dat it creates with your old one and open Bitcoin Core. It should open your old wallet automatically.
If you don't want to download and sync Bitcoin Core, then you could use a program such as pywallet (
https://bitcointalk.org/index.php?topic=34028.0) to dump the private keys which you can then check for balance.
If you encrypted the wallet.dat file with a password, you will obviously need to remember it. If you can't, then you'll need to look at using a program such as btcrecover (
https://github.com/3rdIteration/btcrecover/) to attempt to brute force it.