Help with Exif metatags

[Workaholic]

Basically I have a full set of close to 5000 photos taken from 4/5cameras, and 3 mobiles (if I'm right) from my Summer holiday and after sorting them out manually into different folders did I realise that the "Time Picture Taken" tag is out on each set of photos by a few minutes. Quite a messy mess there then if I want to organise that way, in order that they show the photos in order of how we moved around.

Essentially what I need/want to do is edit (hopefully batch edit) each sets of photos so that the relevant tag is corrected. Ideally I need a freeware based GUI interface that runs on Windows XP (I know there's iPhoto on Mac but I unfortunately don't have access and as my Dell Mini's SSD is bust I can't mac-hack it....) Failing that I can fall back to Ubuntu but running that from a thumb drive (and reading the files from a USB connected drive) on my kinda-bust netbook, the conversion/editing will take a LOT longer.

Secondly I'm a tad puzzled. Wiki mentions that the tags are typically stored at the start of the JPEG files hence any tag changes may corrupt the image, is this right? Also other software packages (Adobe Photoshop I believe) re-compress the file on save hence creating a slightly inferior image, again is this correct?

[TheAnimus]

hmm, I thought Faststone might be able to do this but it can't batch change the EXIF....

Its an odd one. I can only think to write some code to do it, are they all JPEG?

If you don't mind the code idea, download "C# Express Edition" (Free), then use this library:
http://www.codeproject.com/KB/graphics/exiftagcol.aspx or http://code.google.com/p/exiv2net/
(avoid the default ones, as they will try and modify the image data iirc)

If you need help joining the dots, post it on the software forum bit and I'm sure someone will help.