My acer aspire 5745dg is very slow what should i do?

Well..The slow down of computer depends upon many factors viz. virus attack, highly graphical games, heavy software applications etc. By the way, 2 GB RAM is quite less as according to 640 HD. You need to scan the whole system with ab updated antivirus viz. Avast! or MSE, for the virus check up. Install PC Optimizer on your Laptop and also do Disk Defragment.

so if i upgrade to 6 GB RAM or 8 GB RAM my problem would be solved?

i had a similar problem with my acer. so a friend told me to install a cleaning software. he recommended me this one: http://www.simplitec.com/gb/power-suite/ . does anyone know wheter this is a good idea? or is there anything else i can do?

According to your Specs you should have 4Gb RAM, though you won't be able to utilise it all for programmes as some is reserved for your GPU. An upgrade will help, and check your Systray for running software that you don't need...