Thank you so much.
Will the startup repair not interfere with 2nd, theoretically running but not accessible installation?
I found a tutorial on BCDedit here at sevenforums
BCDEDIT - How to Use.
I've used
Used bcdedit /create {ntldr} /d "Windows XP"
bcdedit /set {ntldr} device partition=F:
bcdedit /set {ntldr} path \ntldr
bcdedit /displayorder {ntldr} /addlast
to create another entry, worked so far, just that I get an error trying to access it. Will continue try with the repair disc.
//edit: The repair disc did it, can access both installations now. Problem solved.