That your PC becomes slower, even if you are using CCleaner is very likely because the software you are using is becoming more resource consuming. Well known examples for this are Firefox and Thunderbird. This software tends to eat up all the memory (RAM) and then your PC starts to use the hard disk to temporarily store currently unused RAM on it (so called swapping). This swapping causes your PC to become verrrry slow, as everytime your PC tries to access a memory region thats on the hard drive it must be read from the hard drive first and reading it from there is very slow. Until this has happend the program hangs... Have a look at your PCs hard drive LED. If it starts "blinking" a lot when your PC is slow, then your RAM is full.
Here are some more tricks how to reduce memory consumption of Firefox: https://support.mozilla.org/en-US/kb/firefox-uses-too-much-memory-ram
To know which programs eat up your CPU-resources and RAM you should start the Task Manager. Right-click on your taskbar and select "Start Taskmanager" (or use Ctl+Alt+Del and then choose that option; or Look up "Task Manager" in the Startmenu). There you can sort the Programs by "used memory" or "CPU consumption".
1. An SSD is like a very fast Hard Disk. It's more or less like a big flash drive (same technology), but uses a lot of flash memory in parallel, this way its read/write rate is much faster compared to a usual "usb flash drive". So all your data is loaded/stored faster. Also programs can be loaded/started faster this way. For comparison: A normal hard drive has a read/write speed of about 30-60 MB/s, a SSD has about 200-400 MB/s.
2. SSD dont have a rotating disc like a hard drive, they can just "address" a memory block and read it. Therefore, they don't need to wait until the disc is in a special (rotation position) and also don't need to move a read/write head physically. All this causes waiting times when reading/writing data from a hard drive. Especially if you have a lot of small files or your files are very fragmented and distributed all over the hard drive. In that case your read/write head needs to move long distances to read every part of that file instead of just continuously reading data that is physically next to each other on the drive. SSD do not have problems with fragmentation. It doesn't make a really noticeable difference if your data is very fragmented anymore, even your read/write rate might drop a bit.
Drawback: SSDs have a limited number of write cycles per memory region. Writing a block too often (~100'000 times) causes a sector to fail. Newer SSDs have build in enough spare blocks, that are then used instead. The controller notices the defects and then just uses the other (spare) block. Some cheap SSDs (especially from OCZ) often fail. The problem: This mostly happens "at the moment". So the drive just doesn't work anymore... The data is then "lost" (or you'll need to take that drive to a data recovery service -> expensive). In most cases the SSDs Controller-Board is defective and the SSD can be repaired by replacing the Board (e.g. if you have SSDs of the same type). So you should make backups of your data more often!!!
If you have a normal PC you can also use SSDs in a mirror-RAID or use even more of them with other RAID-levels (e.g. 6 SSDs => usable space of 4 SSDs, speed-up: 4x read/write rate, up 2 discs may fail). The latter case might be interesting if you need lots of storage, as larger SSDs are very expensive. But combining multiple small SDDs can give you a higher speed and the same capacity for a lower price and more safety due to RAID...