IDE "Unable to locate source file,"

Hello there,
I hope someone can help,
I am running BCB4 Standard on W2000 machine, and it has just started getting
a strange message when stepping through code, particularly when converting a
char to an AnsiString eg:

AnsiString SomeFunction(AnsiString TheText);

char SomeText[] = "This is a string of Text";
SomeFunction(SomeText);

I will then get a "Confirm" box:
Unable to locate source file,'DSTRING.CPP.' Ignore this file?

I searched the system, DSTRING.h exists but not cpp.
I also get the same with a different file type: system.pas and another .pas
file that I can't remember.
Any Ideas, as it makes stepping through debugging a tad difficult.
Cheers
Regards
David