UnAuthorised modification of AvastSvc.exe

I get this message every time I start up my PC. It tells me if I continue it could be bad for my system, if I don’t continue then Avast is not operational. I have tried to redownload it a couple of time but still get this problem. Any ideas please.

Welcome to the forum.

What application is giving you this message? (Look at the top bar of the dialogue box that contains the message.)
What OS is it, and what other security apps are running, and what previous AV was used?

Has Avast worked ok before, or is this a new install?

Hi, Thanks for responding to my problem.
The full message I get is-
Avast! detected unauthorised modification of this program file
AvastSvc.exe
Continuing can be dangerous.
Run the program anyway, Yes or No

I have no other antivirus running as I have only just started to use this computer on-line and the first thing I did was to download this program. It found over 400 threats which it deleted. Then I couldn’t start up my windows as it locked up during setup. I did a repair-install of windows to fix the problem and that’s when I started to get the problem as windows asks to select user account.
I’m running XP home edition as my OS.

I hope this will enable you to suggest a solution.
Thanks in advance,
Kevin

It found over 400 threats which it deleted.

Check your computer for Malware with

Malwarebytes Antimalware http://filehippo.com/download_malwarebytes_anti_malware/
after install click UPDATE and run cuick scan, click on REMOVE SELECTED to quarantine anything found

SUPERAntiSpyware http://filehippo.com/download_superantispyware/
Are cookies really spyware and are they dangerous?
http://www.superantispyware.com/supportfaqdisplay.html?faq=26

If anything is found come back and post the scan logs here