Very slow system

I have a Dell Vostro 3555 laptop with an AMD A6-3400 CPU, 3.GB RAM, 64-bit operating system. Over the last couple of months, the machine has gotten slower and slower. I first noticed an increase in the amount of time for the computer to start up, and now the lag affects every application. Based on what I have read in previous threads, the first place to start in correcting this issue is to examine the system start up. One question I have is what the minimum processes that should be allowed at system start up? This is a fairly simple machine, that primarily only runs an antivirus program, Microsoft Office, and Google Chrome.

Any suggestions or ideas on how to improve performance of this very slow computer are much appreciated.

Thanks for your prompt reply. I did run the Malwarebytes scan twice. The first scan did identify some files. The second scan came back clean. I do notice an improvement in system performance. Would additional scans be beneficial?
Regarding system start up - are there any guidelines to follow as to what programs should be included/excluded?

Performance is much improved, however still sluggish.
I was able to create a jpg file of the start up screen, but am having a problem copying it to this screen. Any suggestions?

thanks

Any suggestions for how to reduce or eliminate the sluggishness? I've tested other computers that use the same router/modem and those machines do not have the same issue, so I'm pretty sure it is not a communication issue.