An SSD will do nothing to address the issue you have described. We set up a 6 week test situation in our office with 5 users and 2 randomly assigned laptops. One had an SSD + 7200 rpm HD the other had a 7200 rpm SSD .... no one could ever tell which was which. SSDs do great benchmarks but no one ever typed an extra memo in a day because they got an SSD. A 5400 rpm will load things a bit slower but let's focus on your stated problem:
lately my system keeps crashing and my laptop has also slowed down.
Crashing could be cause by corrupted files or bad RAM tho unlikely. Neither your RAM nor HD has "gotten tired" and slowed down. So your problem is not associated with your hardware (unless it failed somehow) since performance was fine and now it is not and these have not changed. Your problem can be easily identified as follows:
1. If the system is crashing and you are not getting a blue screen / error messages, then you need to disable "automatic restart". This should allow you to view the error massage after next crash
2. You can look in "Event Viewer" to see the case of the crashes as these are recorded in the Windows System Log.
3. One of the most likely causes is you are letting Windows Update install hardware drivers, this should be disabled and all drivers replaced with those on your hardware vendors web sites.
4. Some setting somewhere in Windows has been fudged... best route here is to backup your files and restore the lappie to the "factory condition".
5. A virus or malware is possible. Many of the free utilities will catch a significant number of these but the term "you get what you pay for" applies here. You can do a much more thorough examination with one of the major subscription AV / Malware Suites from Kaspersky or BitDefender. Yes, they cost money but they do have 30 day free trials which should be plenty of time to see if anything is present. When a system is left on my workbench with a suspected infection, this is what I do on the users PC ... most of these have Defender, Spytbot, Adaware, MalawareBytes and others installed. On one that relied on just Wndows built in stiff, we found over 1200 infected files.
Again, all indications point to a Windows problem. I'd say chances are better than 50-50 that 1 or 2 finds the problem.