how to remove files from H.D. securely?

You didn't give a whole lot of information in your post so I will just try to cover all the bases here:

For one, why would even bother to send your computer to a company which you cannot trust? But I understand and appreciate your yearning for privacy and security! OK. What component of you laptop is it that needs to be fixed exactly? If it's something like the screen or whatever other part, then why don't you just grab a screwdriver and remove the hard drive from the machine altogether! Even if the company needs to run on your machine, they can provide their own hard drive or OS! I wish you would have given more information such as your OS and file system or even what kind of hard drive since SSD is a whole different playing field, because yes, it makes a huge difference! Lets assume you are using Windows, NTFS (file system) and a standard HDD. Some file systems, like NTFS, use a feature known as journaling. While journaling is very useful for a number of reasons such as data integrity it's bad for someone who wants to securely delete ALL data remnants from the disk without having to wipe the whole disk! When you delete a file the conventional way, all that is actually happening is the 'link' to the data physically stored on the disk is removed and that area of the disk is marked so that it can be overwritten. Until the data is actually overwritten, it can easily be recovered with a number of methods/tools. Once the data is actually overwritten, you might as well kiss it goodbye for all practical purposes. What a file shredder will do is remove the 'links' then overwrite those bits a given number of times, usually with random data then all zeros on the last run. But if you have a file system which uses journaling, like NTFS, it is still technically possible to recover some of that data. Solid states drives are much better about secure and permanent deletion.

1. Find a company you can trust, but still take care to secure sensitive data.

2. consider encrypting your sensitive files and in the future encrypt your whole disk and make sure to use a good key (at least 9 characters, no dictionary words, upper and lower case, one or two special characters and numbers!)

3. If the company you are sending the machine to isn't doing work on the hard drive then just remove it before you send it in and voila, problem solved.

4. If you still want to delete stuff, make sure you overwrite that data one or two times to make sure it isn't easily recoverable.

5. Be aware that some data might still be accessible with common tools (file carving, etc) even after you do a "shred"

Easiest thing to do would be either to pull the drive out, like I mentioned two times previously or just properly and securely encrypt any and all sensitive data.

I use the open source freeware File Shredder.  Get it from http://www.fileshredder.org/   If you want to securely wipe the free space on your hard drive as well (to get rid of remnants of non-securely deleted files), then get Eraser from http://eraser.heidi.ie/

well after backing up all your personal data, if you want to delete then there is software that will delete it, you need to download Hiren Boot CD 13 and then burn the software, then run as boot from cd and then there is program in Linux then you need select wipe that will wipe your hard drive........