I have Debian. I want to run Windows on the same hard drive, but in separate partitions. I don't want to start my computer "DualBoot" (DualBoot is causing problems for the computer). what should I do? Can I press a button (choose Windows partition) at boot and enter Windows?
Even you did not describe any error niether a problem, I can judge it and try to advice.
First of all, there is no reason for dual boot, if you have hypervisor capable CPU/motherboard, while you can run MS-Windows OS in the Virtual Machine (VM). Running Debian Linux as the host OS gives you at least two systems for virtualisation. The Linux native KVM-Qemu or Qracle (SUN) VirtualBox. Those are reliable, stable and full featured virtualisers: VBox for beginers, KVM-Qemu gives more features and if well configured you can start VM on a single click. It must be said theat Debian is running as the host machine all the time, hence if running MS-Windows in VM you can anytime immediately switch to Debian and vice versa.
If you insist on DualBoot (= start alternatively either Debian either MS-Windows) you definitely must install the MS-Windows first, because if MS-Windows installer detect any other (namely Linux) OS, you cannot continue in installation. Also the Windows-boot-manager does not have the possibility to add any other non MS OS into it's "menu".
If you succesfully install MS-Windows into a sufficient partition on a disk, then you can start the installation of Debian Linux. Just before the end of installation the Grub bootloader is installed, it detects all other preinstalled OSes (MS-Windows in our case), and add them into it's boot menu.
In case of dual boot and namely while Debian Linux is already installed, I prefere to install the MS-Windows OS on a separated harddisk (while the HD with Debian is temporarily disconnected). After the installation of MS-W is finished, Connect the Debian HD back again, configure the BIOS (or UEFI) for boot Debian first, then in running Debian run
update-grub as root what will detect the MS-Windows OS installed on the second HD and add the menuentry to the Grub boot menu.