Most common causes of a slow PC/internet

1  Virus : need no introduction
     Solution : Anti virus

2. Wares
     Malicious software, these are different from virus  and needs same attention. They
     can be broadly grouped  as : Malware, spyware, adware, keyloggers etc
     They can also eat your internet bandwidth significantly causing slow speed
     Solution : Anti malware

3. Hard disk errors
     Power failure or corrupted file/software may be the cause
     Solution : Run check disk utility

4. Fragmentation
     Over time we delete/copy/paste, this causes un-adjacent data in hard-disk
     Solution : Defragment using good defragmenter like smart defrag

5. Less free space on hard disk or its any partition
     Solution : Atleast 15 % space should be free in every partition

6. System restore data
      Restore points are made by windows by default upon (un)installation
      of programs. They may cause slow speed
      Solution : Turn off system restore or decrease its space

7. Junk space
     Over time many unwanted files settles on hard disk
     Solution:Delete them using Revo/ advanced system care/ advanced disk cleaner

8. Registry errors
     When we uninstall any program, its leftover data still
     remains in form of registry
     Solution : clean using registry cleaners

9. Start up programs
     Eats lot of boot time and slow speed
     Solution : Delete unwanted start up program settiings using ccleaner