Anvil Studio

Very lightweight software. I don't remember how I found Anvil Studio but I eventually bought the paid version after using the free version for many songs. If you haven't written music before, it may feel challenging when you see the interface. I didn't use it for weeks after first installing and trying it out, as I only play music by ear. That's how I write music now too: dragging notes around, lengthening them or shortening until it fits the song that I've made on piano. I use Anvil in order to have the songs I make saved in a "permanent" form before I forget the melodies. After they're saved I can come back and work on them later. So maybe you could try it out and see how things work. It is a good product.