1. is your anti virus, anti spyware and firewall up to date?
malware is a leading cause of sudden performance issues.
2. for general clean up
CCleaner is good, another program is Glary utilities
3. also defragment your hard drive, a great tool for that is Piriform's (the makers of ccleaner) Defraggler
unlike windows defragmenter, it lets you choose one file, a group of files or what ever combo you want to defragment and works while you use your pc.
4. make room on your hard drive, the more files you have, the more files the pc is trying to keep track of. get an external usb 2.0 hard drive, transfer files you don't access often to it then disconnect it. that way they are right their if you need them but your O.S. isn't bothering with them.
5. uninstall any programs you don't use. you tried out thunderbird but didn't like it? uninstall it.
remember when you first got your computer and it was really fast? half the reason it was so fast was it didn't have that much in it. Revo uninstaller is the tool to use as it not only uninstalls the program like add/remove does, it also finds and removes registry entries, left over short cuts and everything else that programs bury in your system when they are installed.
6. is your hardware clean and health? look at the back of your pc, is the fan covered in dust? pop the hood, is your motherboard and other internal equipment looking like an egyptian tomb?
that dust and dirt hold in heat, generate static electricity and generally disrupt the smoth operation of your pc.
also if you have your pc in an in closed space like a cabinet under your desk, it may be smoothering in their.pc's are very sensitive to overheating and can die if they get to hot.