What I can add here...
First, when I'm faced to Win7 on 1Gb-RAM PC - I suppose to begin with switch off some autostarts.
And the primary target is Windows Defender - it doesn't provide ANY real defense here while being a big resource hog!
Whether you are using some AV-suite (I recommend Forticlient or BitDefender Free) or not - you don't need Windows Defender. Go to Services and stop it - then set it to disabled.
If your AV-suite has inbuilt firewall - aslo stop and disable Windows Firewall. You don't need two. (My humble opinion is that you don't need even one - but it's up to you).
Also I usually do one big system update and switch Windows Update and BITS services off.
Look for other unneeded services to stop and disable: wireless setup if you use wired connection; WIA - if you don't use scanners; touch options - if no touchscreen present; smart-cards if you don't use them (I'm sure - you don't!
) etc. Google (or search this site) for safe services setups, examples, how-tos and explanations if you need them.
Security: trying to save resources I suggest to refrain from AV-suites. Use EMET for hardening against exploits, SecureFolders for exe run control,
AnVir Task Manager for autostart, processes and services control, and management
and other useful things; Filter web content with K9Web Protection or OpenDNS or Privoxy, use AdBlock/uBlock/AdGuard/AdFender (some of these filtering tools are incompatible though - so you cannot use K9 along with AdFender but most other combination do the job fine).
Use non-mainstream applications for web-related activities: browser, e-mail client, messenger, PDF-reader etc.
Communications: I use Trillian with skypekit instead Skype itself (this way lacks of videocalls, but saves resources and keeps you more secured; you can use it as resident app and load Skype on demand for video.). Google for 'Trillian 5.6 + Skype Kit'
Browser: I've made tests that show that
K-Meleon is the most lightweight among modern actual browsers. And its very secure, capable and customizable one so you can trust me that you cannot find better for low-end PC. Or make your own tests.
Chrome eats all free RAM (which is rest after Win7 core load) with only 5 tabs. And every extensions make the end closer ;-)
Also autostart question is the reason not to use Chrome, Adobe Reader, Java etc. as all of them add their update services in auto position.
Avoid such apps and replace them by alternatives (K-Meleon, Foxit...). The same goes for Flash Player - I suggest to add portable ESR-build of flash np-plugin (unless you use Chrome or its clones already containing their own).
Get sure you really need .NET. Even if you do - I suggest to uninstall your current .NET 4 copy with all the updates got for it; then download the latest package of .NET 4.6 - and reinstall it. This should make your WinSxS folder more compact.
MS says that you can run any .NET app having only last .NET package (some file editing may be needed for some old .NET apps).
Moreover, you may need to make full cleaning of WinSxS folder if you are limited in hard disk space. Use
KB2852386 to learn how to do this.
Office: I can state that SoftMaker's
FreeOffice is the most lightweight option. You can also consider such tools like Atlantis Word Processor (non-free but extremely lightweight Word alternative).
Also I can recommend Sylpheed as 'frugal' e-mail client.
Media: I use old good
Apollo for audio (XmPlay is good alternative). And suppose
CherryPlayer for complex media handling (audio, video, webradio streams, Youtube stuff and vk.com audiocollection).