Best SSD for Parents PC

So last time I was home my parents complained to me that their PC was getting a little slow particularly on startup. i went through and cleaned out some files and disable some startup programs but of course the biggest bottleneck is the 5400RPM WD Green it has. 

I figured I'd throw in an SSD and upgrade to Windows 8.1 while I was at it but I am a bit unsure about what SSD to go with. I want something around 120GB so there'll be enough room for the OS and some programs. It doesn't need to be insanely fast just decently quicker than the HDD. The PC only has SATA II (I think) so I'll be limited there. 

I was thinking the Crucial MX100? I want to stay away from the SSDNow! drives which can be unreliable and aren't much faster than a good 7200RPM HDD. I'd prefer to avoid SandForce all together. Although Amazon does have a good deal on the HyperX 3k.

I'd like to spend less than $70-80. preferably less


Also considering I'll be installing Windows 8.1 on the SSD how would one go about removing Windows 7 from the old HDD while keeping the programs and documents? Is that even possible? I've never done it so I'm a bit in the dark. 

Sandisk Extreme Pro, comes with 10 year warranty.

It id a nice SSD, but a bit much for their needs. It is way faster than SATA II would really allow so it would be nerfed and I don't need a 10 year warranty. Expensive too. 

120gb for $100? You can get just about any ssd you want. The Sandisk is within the price range, and all ssds are going to be Sata II, so I don't see what you are complaining about. Just grab whatever one catches your eye. Literally any of them will work.

Yeah I wanted something under $100. I brought up the SATA II thing because I don't need the extra speed. So I can go with a cheaper drive. 

I suppose the Adata SP600 (LINK)is quite good for the money, I have a 64GB version of it, no problems at all.