As per my knowledge, I don't think there are any utilities which will tell you about the health of the CPU, video card, or the motherboard. If you feel that one of the hardware is failing, then you will have to diagnose it yourself.
There are utilities though for monitoring the health of a hard drive. Kendall has already given you the link for the article. HDD Health, and Active Disk Monitor can show you about the hard drive's status and health.
SpeedFan can show you about the temperatures of various zones like CPU, and hard drive. RealTemp can also show you CPU temperatures.
But to find out if the hardware like CPU, or video card, or motherboard, or power supply is failing, you will have to perform diagnostics other than softwares.
If the PC ran fine before, but runs slow now, I don't think it might be any hardware problem... it might be more of software problem. Please give more details about your PC... like the amount and type of RAM, CPU, OS, hard disk space with free space.
Did you run hard disk defrag? Have you checked if there are unnecessary startup items in the startup list? Have you checked if there are unnecessary processes running which can take up the memory? These things can make the PC slow too... other than virus/spyware infection.
What antivirus do you have? What all scans did you run on your PC... I mean what antispywares or other softwares you used for the scans?
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Anupam
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