Could be a couple different things going on here. Adobe lightroom is not a particularily small program, so depending on where you downloaded and saved it, then extracted it, you may have potentially managed to fill up your hard drive (c:\) If you can get into the computer far enough to check that, you should be able to delete something and alleviate the problem if that is it. Another possibility is that you now have too many startup programs running and you are exceeding the amount of ram or processing power you have available at startup. This can be a particularly bad issue if you are running on a single core processor. 4GB ram is also really low for running windows 7 these days. To check if you are maxed out on resources, press Ctrl+alt_del and select "start task manager" and click on the "performance" tab. If you see CPU or Memory usage pegged at 100% for a long period of time, something is using up all the resources, and your computer is unhappy because of it. If this area looks normal and you don't see high usage, that would be indicative of hard drive issues of some sort.
If you do see high usage, go to the processes tab, and click on the CPU heading listing to sort by the highest resource users. At idle state you should see "system idle process' using most, if not all of the processor. If you see something else using a large portion of those resources, or multiple somethings using all the resources, that can be indicative of a virus or just normal operations not having enough available horsepower to process. Do the same thing for the Memory header and see if anything in particular is latched on to a large chunk of memory at startup (anything bigger than 35,000k is worth looking at, or if you see lots of smaller things that appear to be named random character garbage names like jhsgejlghsjdbg.exe, or other multiple named files. Svchost.exe may be in there multiple times, and a few of them aren't going to be a problem. if you see 20-30 of them though, you may have an infection.
Depending on the age of this system, you could also be running into hard drive failure. If this is an older than 3 years old computer,. I would power it down, take it into the repair place, and ask them to run a hardware scan on the drive and check if it is intact. Extremely slow booting followed by a lockup can be the first stage of hard drive bearing failure. If that is what is occurring, you have a very limited amount of run time remaining on that drive, and should concentrate your efforts on ensuring you have backup copies of any important files and that those backups are not stored on the same hard drive.
if you can get it to run, Speedfan (direct download link: http://www.almico.com/speedfan449.exe ) has a hard drive SMART error monitoring piece. Install that, open it, and click on the SMART tab, then select your hard drive from the drop down list (for a laptop there will usually only be one) then look at the two rating bars at the bottom for "fitness" and "performance". if either of those are lower than 92%, immediately shut off the machine and let us know here. Less than 92% means your hard drive is dying and you need to take the drive to a specialist who can preserve your data if there is anything on that drive you don't already have backed up. If it checks out in the above 95% range, you probably aren't dealing with a failing drive hardware issue.
Ram stick can also go bad over time. if it isn't using all the resources and still keeps locking up, you may have a bad stick. Usually the symptoms of that are "the computer is running along fine and then all the sudden Blue screen!". Slow boot and run can be a symptom of that, but its less common.