Sometimes smaal rectangle shapes appears all over the screen while working, then we have to switchoff the system to reappear the screen

This a freezing of the system caused by the small memory (Ram memory ) or too many programs running in the same time for the capacity of the Ram ..or the CPU ..or just one faulty program or application ...Freezing the screen can be also a issue of running a program that requires video card resources much larger than available ..You should pay attention on the programs running often when the freezing is happening ..and than search on the internet for minimum requiring resources for running that application ..If the system permits you should upgrade..at least the Ram memory for to avoid this happening ..or use similar application with less needs from the system ..
Another cause can be the system being infected and in this case you should reinstall the operation system ..(this is probably the cause if your system has at least 1G of ram ..and 2G CPU ..cause in this case it is enough to run common application of any kind except complicated video ones..and the cause for freezing is a virus )