up until very recently my vaio was running really fast, programs opening quickly. I cannot haven't installed any dubious software as am really careful and only install stuff from trusted sites.
Celeron M processor
160GB HDD with 100GB used
using 2 monitors as extended desktop
8 USB devices, they don't all run at the same time, just leads for various devices for copying files etc
all i use the laptop for is internet and copying files
everything is getting slower and slower
Note: there might be your are under "low performance" mode, switch it to "high performance" and it might help resolve the issue easily.
If this is not the cause, then it can be caused by many reasons such as outdated hardware, overload startup programs, corrupt registry issues, too many junks, virus attack, poor security software taking too much system resources, remnants of uninstalled applications etc. The fact is, you should keep some system maintenance job regularly. See this site sony vaio slow for more fix info.
Meanwhile, upgrading hardware configuration such as add RAM, change a new video card, larger-volume HDD etc can be a fast yet effective way to improve your pc performance. But that won't be the most economical.