What operating system are You running?
What do You use this PC for? A gaming system needs a lot more video card and a lot less RAM than does a server hosting virtual machines.
If it's Microsoft Windows, I would run the Windows Experience Index (WEI.) Right-click on "Computer" and it will be there. While there are some definite shortcomings with WEI, it is free, pre-installed, and easy to use.
WEI will gill give You a set of numbers from 1.0 to 7.9. If one or two are much lower than the others, it may be time to replace some parts (or at least update some drivers. I saw a system score 1.0 on both graphics measurement because the video drivers had never been correctly installed.)
1.0 means something is wrong.
3.5 to 4.0 is an aging hyper-threaded, single-core Pentium 4 or Pentium M.
5.0 to 5.5 is a typical new PC.
7+ is a high-end enthusiast machine.
The biggest problem with WEI is that it only tests the boot drive, and then gives an artificial max of 5.9 to any spinning hard disks. A 7200RPM 5-year-old SATA boot drive will score the same as a RAID-0 of 3x 15.2k SAS drives. It ignores the fact that I move my virtual memory to a SSD and more than double the speed of my PC by doing so.
Another problem is that the graphics measurements are not specific to what you are doing. I have seen one graphics card (ATI FirePro v5800) that is 0.5 "better" in WEI run 10x faster than its competitor (Nvidia Quadr FX1800.)
Finally, WEI does not score how much RAM or disk space You have. Sometimes the amount of RAM is more important that the speed of the RAM.
One last thing...all benchamarks will be "heavy" for a few minutes. If that crashes the system, you have a problem.