Windows 7: Very slow system

Whenever I try to open a programme, my browser, or email software it takes upwards of 45 seconds for the programme to respond. In the wait period the Aero appears rotating then dissapears, then re-appears several times at erratic intervals.
I thought I would backup the registry before trying any solutions only to find that the registry editor does not respond. Could this be a part of the same problem?
I have a limited knowledge of dealing with software problems but this has me baffled. Where do I start looking for what I believe is a corruption in my OS. Is there any software that I can use to analyze my system to find the problem. I have carried out an install/repair with no difference in performance.
Any suggestions or advice would be much appreciated.

first thing i would do is run a thorough virus scan, and then a malware scan.

then run SFC from an elevated command prompt. if it finds errors, restart and run it again. you may have to do this up to four times: SFC /SCANNOW Command - System File Checker

then run disk cleanup: Disk Cleanup - Open and Use

then install and run ccleaner with default settings (make sure to run the registry cleaner repeatedly, until it finds no errors): CCleaner - Standard

Thank you for your helpfull suggestions, all of which I have implemented. None of the scans disclosed any problems though disk clean did remove about 1gb of files. This is from a large disk so I don't think it is sginificant.
Unfortunatly, this has not resolved the problem. Does anyone have any further advice please?

Make sure your disks are not running @ PIO mode.

Check optional Windows Updates for newer drivers. Look in your Device manager to see if any drivers are in error, update or change them.

Study the Event Viewer>Custom>Admin View to google repeat errors to learn about them and how others have resolved them.

Check also the Performance log on Advanced Tools page accessed by clicking your WEI score at Computer>Properties link. Look for cued issues at top of Tools Page, Generate A System Health Report.

Type "solutions" in start search box, see if any Solutions have been sent to problems reported or not.

Test your RAM using memtest86 CD for 5-6 passes, or overnight to really stress test it.

Test your HD using maker's full diag/Repair CD scan: Hard Drive Diagnostics Tools and Utilities (Storage) - TACKtech Corp. followed by Disk Check

Is this a clean install or a factory install of how many years? Consider a Repair Install as a last resort before a Clean Reinstall which is better than getting a new computer - some tips on getting a perfect install based on hundreds I've done are here: re-install windows 7

You didn't give your machine specs?