Do you have enough RAM, if not upgrade.
Make sure there are no viruses
examine the HDD, most often the reason for freezes and lags can be the fragmentation of the drive, if its very high, almost every operation you mentioned gets slow. Its like when you are affected by some sickness, it takes longer to complete any task efficiently, the same happens to a heavily fragmented drive. It has to work extra hard to read fragmented files. Also make sure there is free space on the drive, else delete junk, clear temp files, uninstall unwanted programs and run disk cleanup.
Check your power supply, check the fans, ensure there is no heat issue and also clean out dust if they are clogging the vents.