The error message when I first run Xoops 2.5.0 is :
Strict Standards: Redefining already defined constructor for class MyTextSanitizerExtension in C:\AppServ\www\xoops-2.5.0\htdocs\class\module.textsanitizer.php on line 52

Strict Standards: Redefining already defined constructor for class MyTextSanitizer in C:\AppServ\www\xoops-2.5.0\htdocs\class\module.textsanitizer.php on line 184

Warning: Cannot modify header information - headers already sent by (output started at C:\AppServ\www\xoops-2.5.0\htdocs\class\module.textsanitizer.php:52) in C:\AppServ\www\xoops-2.5.0\htdocs\install\class\installwizard.php on line 87

Warning: Cannot modify header information - headers already sent by (output started at C:\AppServ\www\xoops-2.5.0\htdocs\class\module.textsanitizer.php:52) in C:\AppServ\www\xoops-2.5.0\htdocs\install\class\installwizard.php on line 88

Warning: Cannot modify header information - headers already sent by (output started at C:\AppServ\www\xoops-2.5.0\htdocs\class\module.textsanitizer.php:52) in C:\AppServ\www\xoops-2.5.0\htdocs\install\page_langselect.php on line 33

Warning: Cannot modify header information - headers already sent by (output started at C:\AppServ\www\xoops-2.5.0\htdocs\class\module.textsanitizer.php:52) in C:\AppServ\www\xoops-2.5.0\htdocs\install\page_langselect.php on line 36

Warning: Cannot modify header information - headers already sent by (output started at C:\AppServ\www\xoops-2.5.0\htdocs\class\module.textsanitizer.php:52) in C:\AppServ\www\xoops-2.5.0\htdocs\install\class\installwizard.php on line 204

Warning: Cannot modify header information - headers already sent by (output started at C:\AppServ\www\xoops-2.5.0\htdocs\class\module.textsanitizer.php:52) in C:\AppServ\www\xoops-2.5.0\htdocs\install\class\installwizard.php on line 206

I have looked through other similar posts and have checked for white space, I have also tried deleting and unzipping again.

You should run the filecheck on your files, as it seems like it might be caused by some changes in your files.

Quick search on Google for this error indicates that it is often caused by space or other characters before or after PHP tags. See:

1) here
2) here

What PHP version are you using?