My biome doesnt work

I suggest you to update to 1.9.0, we don't officially support 1.7.3 anymore.

From past experience on older windows operation systems (I have been through a few trust me) unless your running says XP you should have enough RAM to run this software, Vista and 7 should be able to run it without any issues with the amount of ram a stock computer cam with at the time.

I am not sure if you download a lot of software on the internet but some things to keep in mind is that when installing any kind of software as tempting it is to just click next all the way through the installation process doesn't...

Many software products regardless if its free or not will bundle junk that one can harm your computer or bare minimum slow it down due allowing it to run at startup (startup processes) some safety tips take your time go through it slowly and be wary of any add ons or extra products they though at you, they are sneaking and have come up with so many ways to make people think they have not allowed it to install or find new ways to make sure it gets installed, I have had some that you needed to uncheck the terms of services some just has to skip buttons and some you had to click a button then uncheck some kind of thing.

Always get the product for the official site and don't go looking for pirated copies of music, game or any software as many times it comes with some kind of virus, and trust me from past experience someone that has had to completely wipe a computer more than 10 times for sure due to this happening from just wanting to save a few bucks on a game or two, I can truly say I rather pay for it and not lose my data due to any exotic virus that pops open adds or prevents me from removing it, I had one that would spam adds at me every time I closed the add, a new addition would pop up some blind you and then god, the stories....

Anyhow that's for a different day, advice for now remove some unused software do a defrag on your C:/ drive and a disk clean up never hearts, if you have virus protection that can slow down your computer a bit too, I don't use anything more than defender but I am pretty safe online so if you feel the need to keep it then do for sure but do note that they take up a lot of memory do to how they work and run in the background keeping your OS safe.

I think windows 7 only uses 1GB of ram to run and giving that many OS came with more than 1GB at the time even low-end versions having not enough ram should not be the issue its whats using it. windows 10 uses 2GB I think Windows 8 does too but most modern pc come with 8-16 GB somethings high-end ones come with 32 GB standard for a high end is 16 though.

I had an XP and it only used if I recall for the OS 512 MB and had something like 2GB or so plenty at the time it was made but given that they are pretty much obsolete now, and Windows phased them out a while ago along with Vista which was basically Window 7 specs it uncommon to have people use that version now I could see if you have Vista or 7 in that case you should have no problem if you clean up the computer maybe even reinstall windows or have a service place remove unwanted software if you don't feel comfortable doing it your self. 

As far as having mods on older versions of forge I think 2.0 will have the option to select an older version of forge, I could be mistaken but that's something for Klemen to talk about :)